What’s MySQL ?
MySQL (pronounced “my ess cue el”) is an open source relational database management system (RDBMS) that uses Structured Query Language (SQL), the most popular language for adding, accessing, and processing data in a database. Because it is open source, anyone can download MySQL and tailor it to their needs in accordance with the general public license. MySQL is noted mainly for its speed, reliability, and flexibility. …
2) What is DDL, DML and DCL ?
If you look at the large variety of SQL commands, they can be divided into three large subgroups. Data Definition Language deals with database schemas and descriptions of how the data should reside in the database, therefore language statements like CREATE TABLE or ALTER TABLE belong to DDL. DML deals with data manipulation, and therefore includes most common SQL statements such SELECT, INSERT, etc. Data Control Language includes commands such as GRANT, and mostly concerns with rights, permissions and other controls of the database system.
3) How do you get the number of rows affected by query?
SELECT COUNT (user_id) FROM users would only return the number of user_id’s.
4) If the value in the column is repeatable, how do you find out the unique values?
Use DISTINCT in the query, such as SELECT DISTINCT user_firstname FROM users; You can also ask for a number of distinct values by saying SELECT COUNT (DISTINCT user_firstname) FROM users;
5) How do you return the a hundred books starting from 25th?
SELECT book_title FROM books LIMIT 25, 100. The first number in LIMIT is the offset, the second is the number.
6) You wrote a search engine that should retrieve 10 results at a time, but at the same time you’d like to know how many rows there’re total. How do you display that to the user?
SELECT SQL_CALC_FOUND_ROWS page_title FROM web_pages LIMIT 1,10; SELECT FOUND_ROWS(); The second query (not that COUNT() is never used) will tell you how many results there’re total, so you can display a phrase “Found 13,450,600 results, displaying 1-10″. Note that FOUND_ROWS does not pay attention to the LIMITs you specified and always returns the total number of rows affected by query.
7) How would you write a query to select all teams that won either 2, 4, 6 or 8 games?
SELECT team_name FROM teams WHERE team_won IN (2, 4, 6, 8 )
8 ) How would you select all the users, whose phone number is null?
SELECT user_name FROM users WHERE ISNULL(user_phonenumber);
9) What does this query mean: SELECT user_name, user_isp FROM users LEFT JOIN isps USING (user_id) ?
It’s equivalent to saying SELECT user_name, user_isp FROM users LEFT JOIN isps WHERE users.user_id=isps.user_id
10) How do you find out which auto increment was assigned on the last insert?
SELECT LAST_INSERT_ID() will return the last value assigned by the auto_increment function. Note that you don’t have to specify the table name.
11) What does –i-am-a-dummy flag to do when starting MySQL?
Makes the MySQL engine refuse UPDATE and DELETE commands where the WHERE clause is not present.
12) On executing the DELETE statement I keep getting the error about foreign key constraint failing. What do I do?
What it means is that so of the data that you’re trying to delete is still alive in another table. Like if you have a table for universities and a table for students, which contains the ID of the university they go to, running a delete on a university table will fail if the students table still contains people enrolled at that university. Proper way to do it would be to delete the offending data first, and then delete the university in question. Quick way would involve running SET foreign_key_checks=0 before the DELETE command, and setting the parameter back to 1 after the DELETE is done. If your foreign key was formulated with ON DELETE CASCADE, the data in dependent tables will be removed automatically.
13) When would you use ORDER BY in DELETE statement?
When you’re not deleting by row ID. Such as in DELETE FROM test_questions ORDER BY timestamp LIMIT 1. This will delete the most recently posted question in the table test_questions.
14) How can you see all indexes defined for a table?
SHOW INDEX FROM test_questions;
15) How would you change a column from VARCHAR(10) to VARCHAR(50)?
ALTER TABLE test_questions CHANGE test_content test_CONTENT VARCHAR(50).
16) How would you delete a column?
ALTER TABLE test_answers DROP answer_user_id.
17) How would you change a table to InnoDB?
ALTER TABLE test_questions ENGINE innodb;
18) When you create a table, and then run SHOW CREATE TABLE on it, you occasionally get different results than what you typed in. What does MySQL modify in your newly created tables?
1. VARCHARs with length less than 4 become CHARs
2. CHARs with length more than 3 become VARCHARs.
3. NOT NULL gets added to the columns declared as PRIMARY KEYs
4. Default values such as NULL are specified for each column
19) How do I find out all databases starting with ‘tech’ to which I have access to?
SHOW DATABASES LIKE ‘tech%’;
20) How do you concatenate strings in MySQL?
CONCAT (string1, string2, string3)
How do you get a portion of a string?
SELECT SUBSTR(title, 1, 10) from test_questions;
21) What’s the difference between CHAR_LENGTH and LENGTH?
The first is, naturally, the character count. The second is byte count. For the Latin characters the numbers are the same, but they’re not the same for Unicode and other encodings.
22) How do you convert a string to UTF-8?
SELECT (sandykadam USING utf8);
23) What do % and _ mean inside LIKE statement?
% corresponds to 0 or more characters, _ is exactly one character.
24) What does + mean in REGEXP?
At least one character. Appendix G. Regular Expressions from MySQL manual is worth perusing before the interview.
25) How do you get the month from a timestamp?
SELECT MONTH(record_modified_timestamp) from test_questions;
26) How do you offload the time/date handling to MySQL?
SELECT DATE_FORMAT(record_modified_timestamp, ‘%Y-%m-%d’) from test_questions; A similar TIME_FORMAT function deals with time.
27) How do you add three minutes to a date?
ADDDATE(test_publication_date, INTERVAL 3 MINUTE)
28) What’s the difference between Unix timestamps and MySQL timestamps?
Internally Unix timestamps are stored as 32-bit integers, while MySQL timestamps are stored in a similar manner, but represented in readable YYYY-MM-DD HH:MM:SS format.
29) How do you convert between Unix timestamps and MySQL timestamps?
UNIX_TIMESTAMP converts from MySQL timestamp to Unix timestamp, FROM_UNIXTIME converts from Unix timestamp to MySQL timestamp.
30) What are ENUMs used for in MySQL?
You can limit the possible values that go into the table. CREATE TABLE months (month ENUM ‘January’, ‘February’, ‘March’,…); INSERT months VALUES (’April’);
31) How are ENUMs and SETs represented internally?
As unique integers representing the powers of two, due to storage optimizations.
32) How do you start and stop MySQL on Windows?
net start MySQL, net stop MySQL
33) How do you start MySQL on Linux?
/etc/init.d/mysql start
34) Explain the difference between mysql and mysql interfaces in PHP?
mysqli is the object-oriented version of mysql library functions.
What’s the default port for MySQL Server?
3306
35) What does tee command do in MySQL?
tee followed by a filename turns on MySQL logging to a specified file. It can be stopped by command note.
36) Can you save your connection settings to a conf file?
Yes, and name it ~/.my.conf. You might want to change the permissions on the file to 600, so that it’s not readable by others.
37) How do you change a password for an existing user via mysqladmin?
mysqladmin -u root -p password “newpassword”
38) Use mysqldump to create a copy of the database?
mysqldump -h mysqlhost -u username -p mydatabasename > dbdump.sql
39) Have you ever used MySQL Administrator and MySQL Query Browser?
Describe the tasks you accomplished with these tools.
40) What are some good ideas regarding user security in MySQL?
There is no user without a password. There is no user without a user name. There is no user whose Host column contains % (which here indicates that the user can log in from anywhere in the network or the Internet). There are as few users as possible (in the ideal case only root) who have unrestricted access.
41) Explain the difference between MyISAM Static and MyISAM Dynamic. ?
In MyISAM static all the fields have fixed width. The Dynamic MyISAM table would include fields such as TEXT, BLOB, etc. to accommodate the data types with various lengths. MyISAM Static would be easier to restore in case of corruption, since even though you might lose some data, you know exactly where to look for the beginning of the next record.
42) What does myisamchk do?
It compressed the MyISAM tables, which reduces their disk usage.
43) Explain advantages of InnoDB over MyISAM?
Row-level locking, transactions, foreign key constraints and crash recovery.
44) Explain advantages of MyISAM over InnoDB?
Much more conservative approach to disk space management – each MyISAM table is stored in a separate file, which could be compressed then with myisamchk if needed. With InnoDB the tables are stored in tablespace, and not much further optimization is possible. All data except for TEXT and BLOB can occupy 8,000 bytes at most. No full text indexing is available for InnoDB. TRhe COUNT(*)s execute slower than in MyISAM due to tablespace complexity.
45) What are HEAP tables in MySQL?
HEAP tables are in-memory. They are usually used for high-speed temporary storage. No TEXT or BLOB fields are allowed within HEAP tables. You can only use the comparison operators = and. HEAP tables do not support AUTO_INCREMENT. Indexes must be NOT NULL.
46) How do you control the max size of a HEAP table?
MySQL config variable max_heap_table_size.
47) What are CSV tables?
Those are the special tables, data for which is saved into comma-separated values files. They cannot be indexed.
48) Explain federated tables. ?
Introduced in MySQL 5.0, federated tables allow access to the tables located on other databases on other servers.
49) What is SERIAL data type in MySQL?
BIGINT NOT NULL PRIMARY KEY AUTO_INCREMENT
50) What happens when the column is set to AUTO INCREMENT and you reach the maximum value for that table?
It stops incrementing. It does not overflow to 0 to prevent data losses, but further inserts are going to produce an error, since the key has been used already.
51) Explain the difference between BOOL, TINYINT and BIT. ?
Prior to MySQL 5.0.3: those are all synonyms. After MySQL 5.0.3: BIT data type can store 8 bytes of data and should be used for binary data.
52) Explain the difference between FLOAT, DOUBLE and REAL. ?
FLOATs store floating point numbers with 8 place accuracy and take up 4 bytes. DOUBLEs store floating point numbers with 16 place accuracy and take up 8 bytes. REAL is a synonym of FLOAT for now.
53) If you specify the data type as DECIMAL (5,2), what’s the range of values that can go in this table?
999.99 to -99.99. Note that with the negative number the minus sign is considered one of the digits.
54) What happens if a table has one column defined as TIMESTAMP?
That field gets the current timestamp whenever the row gets altered.
55) But what if you really want to store the timestamp data, such as the publication date of the article?
Create two columns of type TIMESTAMP and use the second one for your real data.
Explain data type TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP ?
The column exhibits the same behavior as a single timestamp column in a table with no other timestamp columns.
56) What does TIMESTAMP ON UPDATE CURRENT_TIMESTAMP data type do?
On initialization places a zero in that column, on future updates puts the current value of the timestamp in.
57) Explain TIMESTAMP DEFAULT ‘2006:09:02 17:38:44? ON UPDATE CURRENT_TIMESTAMP. ?
A default value is used on initialization, a current timestamp is inserted on update of the row.
58) If I created a column with data type VARCHAR(3), what would I expect to see in MySQL table?
CHAR(3), since MySQL automatically adjusted the data type.
59) General Information About MySQL
MySQL is a very fast, multi-threaded, multi-user, and robust SQL (Structured Query Language) database server.
60) MySQL is free software.
It is licensed with the GNU GENERAL PUBLIC LICENSE http://www.gnu.org/.
61) What Is MySQL
MySQL, the most popular Open Source SQL database, is provided by MySQL AB. MySQL AB is a commercial company that builds is business providing services around the MySQL database. See section 1.2 What Is MySQL AB.
62) MySQL is a database management system.
A database is a structured collection of data. It may be anything from a simple shopping list to a picture gallery or the vast amounts of information in a corporate network. To add, access, and process data stored in a computer database, you need a database management system such as MySQL. Since computers are very good at handling large amounts of data, database management plays a central role in computing, as stand-alone utilities, or as parts of other applications.
63) MySQL is a relational database management system.
A relational database stores data in separate tables rather than putting all the data in one big storeroom. This adds speed and flexibility. The tables are linked by defined relations making it possible to combine data from several tables on request. The SQL part of MySQL stands for “Structured Query Language” – the most common standardized language used to access databases.
64) MySQL is Open Source Software.
Open source means that it is possible for anyone to use and modify. Anybody can download MySQL from the Internet and use it without paying anything. Anybody so inclined can study the source code and change it to fit their needs. MySQL uses the GPL (GNU General Public License) http://www.gnu.org, to define what you may and may not do with the software in different situations. If you feel uncomfortable with the GPL or need to embed MySQL into a commercial application you can buy a commercially licensed version from us.
65) Why use MySQL?
MySQL is very fast, reliable, and easy to use. If that is what you are looking for, you should give it a try. MySQL also has a very practical set of features developed in very close cooperation with our users. You can find a performance comparison of MySQL to some other database managers on our benchmark page. See section 12.7 Using Your Own Benchmarks. MySQL was originally developed to handle very large databases much faster than existing solutions and has been successfully used in highly demanding production environments for several years. Though under constant development, MySQL today offers a rich and very useful set of functions. The connectivity, speed, and security make MySQL highly suited for accessing databases on the Internet.
66) The technical features of MySQL
For advanced technical information, see section 7 MySQL Language Reference. MySQL is a client/server system that consists of a multi-threaded SQL server that supports different backends, several different client programs and libraries, administrative tools, and a programming interface. We also provide MySQL as a multi-threaded library which you can link into your application to get a smaller, faster, easier to manage product. MySQL has a lot of contributed software available.
It is very likely that you will find that your favorite application/language already supports MySQL. The official way to pronounce MySQL is “My Ess Que Ell” (not MY-SEQUEL). But we try to avoid correcting people who say MY-SEQUEL.
67) Explain Database Basics
Databases are managed by a relational database management system (RDBMS). An RDBMS supports a database language to create and delete databases and to manage and search data. The database language used in almost all DBMSs is SQL, a set of statements that define and manipulate data. After creating a database, the most common SQL statements used are INSERT, UPDATE, DELETE, and SELECT, which add, change, remove, and search data in a database, respectively.
68) what is Database?
A repository to store data.
69) What is Table?
The part of a database that stores the data. A table has columns or attributes, and the data stored in rows.
70) Define Attributes?
The columns in a table. All rows in table entities have the same attributes. For example, a customer table might have the attributes name, address, and city. Each attribute has a data type such as string, integer, or date.
71) What are Rows?
The data entries in a table. Rows contain values for each attribute. For example, a row in a customer table might contain the values “Matthew Richardson,” “Punt Road,” and “Richmond.” Rows are also known as records.
72) Explain Relational model?
A model that uses tables to store data and manage the relationship between tables.
73) Define Relational database management system?
A software system that manages data in a database and is based on the relational model. DBMSs have several components described in detail in Chapter 1.
74) What is SQL?
A query language that interacts with a DBMS. SQL is a set of statements to manage databases, tables, and data.
75) Define Constraints?
Restrictions or limitations on tables and attributes. For example, a wine can be produced only by one winery, an order for wine can’t exist if it isn’t associated with a customer, having a name attribute could be mandatory for a customer.
76) What is Primary key?
One or more attributes that contain values that uniquely identify each row. For example, a customer table might have the primary key of cust ID. The cust ID attribute is then assigned a unique value for each customer. A primary key is a constraint of most tables.
77) Define Index?
A data structure used for fast access to rows in a table. An index is usually built for the primary key of each table and can then be used to quickly find a particular row. Indexes are also defined and built for other attributes when those attributes are frequently used in queries.
78) Define Entity-relationship modeling?
A technique used to describe the real-world data in terms of entities, attributes, and relationships.
79) What is Normalized database?
A correctly designed database that is created from an ER model. There are different types or levels of normalization, and a third-normal form database is generally regarded as being an acceptably designed relational database.
80) MySQL – SELECT INTO TABLE
MySQL doesn’t yet support the Oracle SQL extension: SELECT … INTO TABLE …. MySQL supports instead the ANSI SQL syntax INSERT INTO … SELECT …, which is basically the same thing.
Alternatively, you can use SELECT INTO OUTFILE… or CREATE TABLE … SELECT to solve your problem.
81) MySQL – Stored Procedures and Triggers
A stored procedure is a set of SQL commands that can be compiled and stored in the server. Once this has been done, clients don’t need to keep reissuing the entire query but can refer to the stored procedure. This provides better performance because the query has to be parsed only once, and less information needs to be sent between the server and the client. You can also raise the conceptual level by having libraries of functions in the server.
A trigger is a stored procedure that is invoked when a particular event occurs. For example, you can install a stored procedure that is triggered each time a record is deleted from a transaction table and that automatically deletes the corresponding customer from a customer table when all his transactions are deleted.
The planned update language will be able to handle stored procedures, but without triggers. Triggers usually slow down everything, even queries for which they are not needed.
82) MySQL – What the Privilege System Does
The primary function of the MySQL privilege system is to authenticate a user connecting from a given host, and to associate that user with privileges on a database such as select, insert, update and delete.
Additional functionality includes the ability to have an anonymous user and to grant privileges for MySQL-specific functions such as LOAD DATA INFILE and administrative operations.
83) MySQL User Names and Passwords
There are several distinctions between the way user names and passwords are used by MySQL and the way they are used by Unix or Windows:
User names, as used by MySQL for authentication purposes, have nothing to do with Unix user names (login names) or Windows user names. Most MySQL clients by default try to log in using the current Unix user name as the MySQL user name, but that is for convenience only. Client programs allow a different name to be specified with the -u or –user options. This means that you can’t make a database secure in any way unless all MySQL user names have passwords. Anyone may attempt to connect to the server using any name, and they will succeed if they specify any name that doesn’t have a password. MySQL user names can be up to 16 characters long; Unix user names typically are limited to 8 characters. MySQL passwords have nothing to do with Unix passwords. There is no necessary connection between the password you use to log in to a Unix machine and the password you use to access a database on that machine. MySQL encrypts passwords using a different algorithm than the one used during the Unix login process.
Note that even if the password is stored ‘scrambled’, and knowing your ‘scrambled’ password is enough to be able to connect to the MySQL server!
84) MySQL – When Privilege Changes Take Effect
When mysqld starts, all grant table contents are read into memory and become effective at that point.
Modifications to the grant tables that you perform using GRANT, REVOKE, or SET PASSWORD are noticed by the server immediately.
If you modify the grant tables manually (using INSERT, UPDATE, etc.), you should execute a FLUSH PRIVILEGES statement or run mysqladmin flush-privileges or mysqladmin reload to tell the server to reload the grant tables. Otherwise your changes will have no effect until you restart the server. If you change the grant tables manually but forget to reload the privileges, you will be wondering why your changes don’t seem to make any difference!
When the server notices that the grant tables have been changed, existing client connections are affected as follows:
Table and column privilege changes take effect with the client’s next request.
Database privilege changes take effect at the next USE db_name command.
Global privilege changes and password changes take effect the next time the client connects.
85) How do I rotate replication logs?
In Version 3.23.28 you should use PURGE MASTER LOGS TO command after determining which logs can be deleted, and optionally backing them up first. In earlier versions the process is much more painful, and cannot be safely done without stopping all the slaves in the case that you plan to re-use log names. You will need to stop the slave threads, edit the binary log index file, delete all the old logs, restart the master, start slave threads,and then remove the old log files.
86) How do I upgrade on a hot replication setup?
If you are upgrading pre-3.23.26 versions, you should just lock the master tables, let the slave catch up, then run FLUSH MASTER on the master, and FLUSH SLAVE on the slave to reset the logs, then restart new versions of the master and the slave. Note that the slave can stay down for some time – since the master is logging all the updates, the slave will be able to catch up once it is up and can connect.
After 3.23.26, we have locked the replication protocol for modifications, so you can upgrade masters and slave on the fly to a newer 3.23 version and you can have different versions of MySQL running on the slave and the master, as long as they are both newer than 3.23.26.
87) How can I use replication to improve performance of my system?
You should set up one server as the master, and direct all writes to it, and configure as many slaves as you have the money and rackspace for, distributing the reads among the master and the slaves. You can also start the slaves with –skip-bdb, –low-priority-updates and –delay-key-write-for-all-tables to get speed improvements for the slave. In this case the slave will use non-transactional MyISAM tables instead of BDB tables to get more speed.
88) MySQL – Drawbacks to Creating Large Numbers of Tables in the Same Database
If you have many files in a directory, open, close, and create operations will be slow. If you execute SELECT statements on many different tables, there will be a little overhead when the table cache is full, because for every table that has to be opened, another must be closed. You can reduce this overhead by making the table cache larger.
89) MySQL – Why So Many Open tables?
When you run mysqladmin status, you’ll see something like this:
Uptime: 426 Running threads: 1 Questions: 11082 Reloads: 1 Open tables: 12
This can be somewhat perplexing if you only have 6 tables.
MySQL is multithreaded, so it may have many queries on the same table simultaneously. To minimize the problem with two threads having different states on the same file, the table is opened independently by each concurrent thread. This takes some memory and one extra file descriptor for the data file. The index file descriptor is shared between all threads.
90) How MySQL Optimizes DISTINCT ?
DISTINCT is converted to a GROUP BY on all columns, DISTINCT combined with ORDER BY will in many cases also need a temporary table.
When combining LIMIT # with DISTINCT, MySQL will stop as soon as it finds # unique rows.
If you don’t use columns from all used tables, MySQL will stop the scanning of the not used tables as soon as it has found the first match.
SELECT DISTINCT t1.a FROM t1,t2 where t1.a=t2.a;
In the case, assuming t1 is used before t2 (check with EXPLAIN), then MySQL will stop reading from t2 (for that particular row in t1) when the first row in t2 is found.
91) How MySQL Optimizes LIMIT ?
In some cases MySQL will handle the query differently when you are using LIMIT # and not using HAVING:
If you are selecting only a few rows with LIMIT, MySQL will use indexes in some cases when it normally would prefer to do a full table scan.
If you use LIMIT # with ORDER BY, MySQL will end the sorting as soon as it has found the first # lines instead of sorting the whole table.
When combining LIMIT # with DISTINCT, MySQL will stop as soon as it finds # unique rows.
In some cases a GROUP BY can be resolved by reading the key in order (or do a sort on the key) and then calculate summaries until the key value changes. In this case LIMIT # will not calculate any unnecessary GROUP BY’s.
As soon as MySQL has sent the first # rows to the client, it will abort the query.
LIMIT 0 will always quickly return an empty set. This is useful to check the query and to get the column types of the result columns.
The size of temporary tables uses the LIMIT # to calculate how much space is needed to resolve the query.
MySQL (pronounced “my ess cue el”) is an open source relational database management system (RDBMS) that uses Structured Query Language (SQL), the most popular language for adding, accessing, and processing data in a database. Because it is open source, anyone can download MySQL and tailor it to their needs in accordance with the general public license. MySQL is noted mainly for its speed, reliability, and flexibility. …
2) What is DDL, DML and DCL ?
If you look at the large variety of SQL commands, they can be divided into three large subgroups. Data Definition Language deals with database schemas and descriptions of how the data should reside in the database, therefore language statements like CREATE TABLE or ALTER TABLE belong to DDL. DML deals with data manipulation, and therefore includes most common SQL statements such SELECT, INSERT, etc. Data Control Language includes commands such as GRANT, and mostly concerns with rights, permissions and other controls of the database system.
3) How do you get the number of rows affected by query?
SELECT COUNT (user_id) FROM users would only return the number of user_id’s.
4) If the value in the column is repeatable, how do you find out the unique values?
Use DISTINCT in the query, such as SELECT DISTINCT user_firstname FROM users; You can also ask for a number of distinct values by saying SELECT COUNT (DISTINCT user_firstname) FROM users;
5) How do you return the a hundred books starting from 25th?
SELECT book_title FROM books LIMIT 25, 100. The first number in LIMIT is the offset, the second is the number.
6) You wrote a search engine that should retrieve 10 results at a time, but at the same time you’d like to know how many rows there’re total. How do you display that to the user?
SELECT SQL_CALC_FOUND_ROWS page_title FROM web_pages LIMIT 1,10; SELECT FOUND_ROWS(); The second query (not that COUNT() is never used) will tell you how many results there’re total, so you can display a phrase “Found 13,450,600 results, displaying 1-10″. Note that FOUND_ROWS does not pay attention to the LIMITs you specified and always returns the total number of rows affected by query.
7) How would you write a query to select all teams that won either 2, 4, 6 or 8 games?
SELECT team_name FROM teams WHERE team_won IN (2, 4, 6, 8 )
8 ) How would you select all the users, whose phone number is null?
SELECT user_name FROM users WHERE ISNULL(user_phonenumber);
9) What does this query mean: SELECT user_name, user_isp FROM users LEFT JOIN isps USING (user_id) ?
It’s equivalent to saying SELECT user_name, user_isp FROM users LEFT JOIN isps WHERE users.user_id=isps.user_id
10) How do you find out which auto increment was assigned on the last insert?
SELECT LAST_INSERT_ID() will return the last value assigned by the auto_increment function. Note that you don’t have to specify the table name.
11) What does –i-am-a-dummy flag to do when starting MySQL?
Makes the MySQL engine refuse UPDATE and DELETE commands where the WHERE clause is not present.
12) On executing the DELETE statement I keep getting the error about foreign key constraint failing. What do I do?
What it means is that so of the data that you’re trying to delete is still alive in another table. Like if you have a table for universities and a table for students, which contains the ID of the university they go to, running a delete on a university table will fail if the students table still contains people enrolled at that university. Proper way to do it would be to delete the offending data first, and then delete the university in question. Quick way would involve running SET foreign_key_checks=0 before the DELETE command, and setting the parameter back to 1 after the DELETE is done. If your foreign key was formulated with ON DELETE CASCADE, the data in dependent tables will be removed automatically.
13) When would you use ORDER BY in DELETE statement?
When you’re not deleting by row ID. Such as in DELETE FROM test_questions ORDER BY timestamp LIMIT 1. This will delete the most recently posted question in the table test_questions.
14) How can you see all indexes defined for a table?
SHOW INDEX FROM test_questions;
15) How would you change a column from VARCHAR(10) to VARCHAR(50)?
ALTER TABLE test_questions CHANGE test_content test_CONTENT VARCHAR(50).
16) How would you delete a column?
ALTER TABLE test_answers DROP answer_user_id.
17) How would you change a table to InnoDB?
ALTER TABLE test_questions ENGINE innodb;
18) When you create a table, and then run SHOW CREATE TABLE on it, you occasionally get different results than what you typed in. What does MySQL modify in your newly created tables?
1. VARCHARs with length less than 4 become CHARs
2. CHARs with length more than 3 become VARCHARs.
3. NOT NULL gets added to the columns declared as PRIMARY KEYs
4. Default values such as NULL are specified for each column
19) How do I find out all databases starting with ‘tech’ to which I have access to?
SHOW DATABASES LIKE ‘tech%’;
20) How do you concatenate strings in MySQL?
CONCAT (string1, string2, string3)
How do you get a portion of a string?
SELECT SUBSTR(title, 1, 10) from test_questions;
21) What’s the difference between CHAR_LENGTH and LENGTH?
The first is, naturally, the character count. The second is byte count. For the Latin characters the numbers are the same, but they’re not the same for Unicode and other encodings.
22) How do you convert a string to UTF-8?
SELECT (sandykadam USING utf8);
23) What do % and _ mean inside LIKE statement?
% corresponds to 0 or more characters, _ is exactly one character.
24) What does + mean in REGEXP?
At least one character. Appendix G. Regular Expressions from MySQL manual is worth perusing before the interview.
25) How do you get the month from a timestamp?
SELECT MONTH(record_modified_timestamp) from test_questions;
26) How do you offload the time/date handling to MySQL?
SELECT DATE_FORMAT(record_modified_timestamp, ‘%Y-%m-%d’) from test_questions; A similar TIME_FORMAT function deals with time.
27) How do you add three minutes to a date?
ADDDATE(test_publication_date, INTERVAL 3 MINUTE)
28) What’s the difference between Unix timestamps and MySQL timestamps?
Internally Unix timestamps are stored as 32-bit integers, while MySQL timestamps are stored in a similar manner, but represented in readable YYYY-MM-DD HH:MM:SS format.
29) How do you convert between Unix timestamps and MySQL timestamps?
UNIX_TIMESTAMP converts from MySQL timestamp to Unix timestamp, FROM_UNIXTIME converts from Unix timestamp to MySQL timestamp.
30) What are ENUMs used for in MySQL?
You can limit the possible values that go into the table. CREATE TABLE months (month ENUM ‘January’, ‘February’, ‘March’,…); INSERT months VALUES (’April’);
31) How are ENUMs and SETs represented internally?
As unique integers representing the powers of two, due to storage optimizations.
32) How do you start and stop MySQL on Windows?
net start MySQL, net stop MySQL
33) How do you start MySQL on Linux?
/etc/init.d/mysql start
34) Explain the difference between mysql and mysql interfaces in PHP?
mysqli is the object-oriented version of mysql library functions.
What’s the default port for MySQL Server?
3306
35) What does tee command do in MySQL?
tee followed by a filename turns on MySQL logging to a specified file. It can be stopped by command note.
36) Can you save your connection settings to a conf file?
Yes, and name it ~/.my.conf. You might want to change the permissions on the file to 600, so that it’s not readable by others.
37) How do you change a password for an existing user via mysqladmin?
mysqladmin -u root -p password “newpassword”
38) Use mysqldump to create a copy of the database?
mysqldump -h mysqlhost -u username -p mydatabasename > dbdump.sql
39) Have you ever used MySQL Administrator and MySQL Query Browser?
Describe the tasks you accomplished with these tools.
40) What are some good ideas regarding user security in MySQL?
There is no user without a password. There is no user without a user name. There is no user whose Host column contains % (which here indicates that the user can log in from anywhere in the network or the Internet). There are as few users as possible (in the ideal case only root) who have unrestricted access.
41) Explain the difference between MyISAM Static and MyISAM Dynamic. ?
In MyISAM static all the fields have fixed width. The Dynamic MyISAM table would include fields such as TEXT, BLOB, etc. to accommodate the data types with various lengths. MyISAM Static would be easier to restore in case of corruption, since even though you might lose some data, you know exactly where to look for the beginning of the next record.
42) What does myisamchk do?
It compressed the MyISAM tables, which reduces their disk usage.
43) Explain advantages of InnoDB over MyISAM?
Row-level locking, transactions, foreign key constraints and crash recovery.
44) Explain advantages of MyISAM over InnoDB?
Much more conservative approach to disk space management – each MyISAM table is stored in a separate file, which could be compressed then with myisamchk if needed. With InnoDB the tables are stored in tablespace, and not much further optimization is possible. All data except for TEXT and BLOB can occupy 8,000 bytes at most. No full text indexing is available for InnoDB. TRhe COUNT(*)s execute slower than in MyISAM due to tablespace complexity.
45) What are HEAP tables in MySQL?
HEAP tables are in-memory. They are usually used for high-speed temporary storage. No TEXT or BLOB fields are allowed within HEAP tables. You can only use the comparison operators = and. HEAP tables do not support AUTO_INCREMENT. Indexes must be NOT NULL.
46) How do you control the max size of a HEAP table?
MySQL config variable max_heap_table_size.
47) What are CSV tables?
Those are the special tables, data for which is saved into comma-separated values files. They cannot be indexed.
48) Explain federated tables. ?
Introduced in MySQL 5.0, federated tables allow access to the tables located on other databases on other servers.
49) What is SERIAL data type in MySQL?
BIGINT NOT NULL PRIMARY KEY AUTO_INCREMENT
50) What happens when the column is set to AUTO INCREMENT and you reach the maximum value for that table?
It stops incrementing. It does not overflow to 0 to prevent data losses, but further inserts are going to produce an error, since the key has been used already.
51) Explain the difference between BOOL, TINYINT and BIT. ?
Prior to MySQL 5.0.3: those are all synonyms. After MySQL 5.0.3: BIT data type can store 8 bytes of data and should be used for binary data.
52) Explain the difference between FLOAT, DOUBLE and REAL. ?
FLOATs store floating point numbers with 8 place accuracy and take up 4 bytes. DOUBLEs store floating point numbers with 16 place accuracy and take up 8 bytes. REAL is a synonym of FLOAT for now.
53) If you specify the data type as DECIMAL (5,2), what’s the range of values that can go in this table?
999.99 to -99.99. Note that with the negative number the minus sign is considered one of the digits.
54) What happens if a table has one column defined as TIMESTAMP?
That field gets the current timestamp whenever the row gets altered.
55) But what if you really want to store the timestamp data, such as the publication date of the article?
Create two columns of type TIMESTAMP and use the second one for your real data.
Explain data type TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP ?
The column exhibits the same behavior as a single timestamp column in a table with no other timestamp columns.
56) What does TIMESTAMP ON UPDATE CURRENT_TIMESTAMP data type do?
On initialization places a zero in that column, on future updates puts the current value of the timestamp in.
57) Explain TIMESTAMP DEFAULT ‘2006:09:02 17:38:44? ON UPDATE CURRENT_TIMESTAMP. ?
A default value is used on initialization, a current timestamp is inserted on update of the row.
58) If I created a column with data type VARCHAR(3), what would I expect to see in MySQL table?
CHAR(3), since MySQL automatically adjusted the data type.
59) General Information About MySQL
MySQL is a very fast, multi-threaded, multi-user, and robust SQL (Structured Query Language) database server.
60) MySQL is free software.
It is licensed with the GNU GENERAL PUBLIC LICENSE http://www.gnu.org/.
61) What Is MySQL
MySQL, the most popular Open Source SQL database, is provided by MySQL AB. MySQL AB is a commercial company that builds is business providing services around the MySQL database. See section 1.2 What Is MySQL AB.
62) MySQL is a database management system.
A database is a structured collection of data. It may be anything from a simple shopping list to a picture gallery or the vast amounts of information in a corporate network. To add, access, and process data stored in a computer database, you need a database management system such as MySQL. Since computers are very good at handling large amounts of data, database management plays a central role in computing, as stand-alone utilities, or as parts of other applications.
63) MySQL is a relational database management system.
A relational database stores data in separate tables rather than putting all the data in one big storeroom. This adds speed and flexibility. The tables are linked by defined relations making it possible to combine data from several tables on request. The SQL part of MySQL stands for “Structured Query Language” – the most common standardized language used to access databases.
64) MySQL is Open Source Software.
Open source means that it is possible for anyone to use and modify. Anybody can download MySQL from the Internet and use it without paying anything. Anybody so inclined can study the source code and change it to fit their needs. MySQL uses the GPL (GNU General Public License) http://www.gnu.org, to define what you may and may not do with the software in different situations. If you feel uncomfortable with the GPL or need to embed MySQL into a commercial application you can buy a commercially licensed version from us.
65) Why use MySQL?
MySQL is very fast, reliable, and easy to use. If that is what you are looking for, you should give it a try. MySQL also has a very practical set of features developed in very close cooperation with our users. You can find a performance comparison of MySQL to some other database managers on our benchmark page. See section 12.7 Using Your Own Benchmarks. MySQL was originally developed to handle very large databases much faster than existing solutions and has been successfully used in highly demanding production environments for several years. Though under constant development, MySQL today offers a rich and very useful set of functions. The connectivity, speed, and security make MySQL highly suited for accessing databases on the Internet.
66) The technical features of MySQL
For advanced technical information, see section 7 MySQL Language Reference. MySQL is a client/server system that consists of a multi-threaded SQL server that supports different backends, several different client programs and libraries, administrative tools, and a programming interface. We also provide MySQL as a multi-threaded library which you can link into your application to get a smaller, faster, easier to manage product. MySQL has a lot of contributed software available.
It is very likely that you will find that your favorite application/language already supports MySQL. The official way to pronounce MySQL is “My Ess Que Ell” (not MY-SEQUEL). But we try to avoid correcting people who say MY-SEQUEL.
67) Explain Database Basics
Databases are managed by a relational database management system (RDBMS). An RDBMS supports a database language to create and delete databases and to manage and search data. The database language used in almost all DBMSs is SQL, a set of statements that define and manipulate data. After creating a database, the most common SQL statements used are INSERT, UPDATE, DELETE, and SELECT, which add, change, remove, and search data in a database, respectively.
68) what is Database?
A repository to store data.
69) What is Table?
The part of a database that stores the data. A table has columns or attributes, and the data stored in rows.
70) Define Attributes?
The columns in a table. All rows in table entities have the same attributes. For example, a customer table might have the attributes name, address, and city. Each attribute has a data type such as string, integer, or date.
71) What are Rows?
The data entries in a table. Rows contain values for each attribute. For example, a row in a customer table might contain the values “Matthew Richardson,” “Punt Road,” and “Richmond.” Rows are also known as records.
72) Explain Relational model?
A model that uses tables to store data and manage the relationship between tables.
73) Define Relational database management system?
A software system that manages data in a database and is based on the relational model. DBMSs have several components described in detail in Chapter 1.
74) What is SQL?
A query language that interacts with a DBMS. SQL is a set of statements to manage databases, tables, and data.
75) Define Constraints?
Restrictions or limitations on tables and attributes. For example, a wine can be produced only by one winery, an order for wine can’t exist if it isn’t associated with a customer, having a name attribute could be mandatory for a customer.
76) What is Primary key?
One or more attributes that contain values that uniquely identify each row. For example, a customer table might have the primary key of cust ID. The cust ID attribute is then assigned a unique value for each customer. A primary key is a constraint of most tables.
77) Define Index?
A data structure used for fast access to rows in a table. An index is usually built for the primary key of each table and can then be used to quickly find a particular row. Indexes are also defined and built for other attributes when those attributes are frequently used in queries.
78) Define Entity-relationship modeling?
A technique used to describe the real-world data in terms of entities, attributes, and relationships.
79) What is Normalized database?
A correctly designed database that is created from an ER model. There are different types or levels of normalization, and a third-normal form database is generally regarded as being an acceptably designed relational database.
80) MySQL – SELECT INTO TABLE
MySQL doesn’t yet support the Oracle SQL extension: SELECT … INTO TABLE …. MySQL supports instead the ANSI SQL syntax INSERT INTO … SELECT …, which is basically the same thing.
Alternatively, you can use SELECT INTO OUTFILE… or CREATE TABLE … SELECT to solve your problem.
81) MySQL – Stored Procedures and Triggers
A stored procedure is a set of SQL commands that can be compiled and stored in the server. Once this has been done, clients don’t need to keep reissuing the entire query but can refer to the stored procedure. This provides better performance because the query has to be parsed only once, and less information needs to be sent between the server and the client. You can also raise the conceptual level by having libraries of functions in the server.
A trigger is a stored procedure that is invoked when a particular event occurs. For example, you can install a stored procedure that is triggered each time a record is deleted from a transaction table and that automatically deletes the corresponding customer from a customer table when all his transactions are deleted.
The planned update language will be able to handle stored procedures, but without triggers. Triggers usually slow down everything, even queries for which they are not needed.
82) MySQL – What the Privilege System Does
The primary function of the MySQL privilege system is to authenticate a user connecting from a given host, and to associate that user with privileges on a database such as select, insert, update and delete.
Additional functionality includes the ability to have an anonymous user and to grant privileges for MySQL-specific functions such as LOAD DATA INFILE and administrative operations.
83) MySQL User Names and Passwords
There are several distinctions between the way user names and passwords are used by MySQL and the way they are used by Unix or Windows:
User names, as used by MySQL for authentication purposes, have nothing to do with Unix user names (login names) or Windows user names. Most MySQL clients by default try to log in using the current Unix user name as the MySQL user name, but that is for convenience only. Client programs allow a different name to be specified with the -u or –user options. This means that you can’t make a database secure in any way unless all MySQL user names have passwords. Anyone may attempt to connect to the server using any name, and they will succeed if they specify any name that doesn’t have a password. MySQL user names can be up to 16 characters long; Unix user names typically are limited to 8 characters. MySQL passwords have nothing to do with Unix passwords. There is no necessary connection between the password you use to log in to a Unix machine and the password you use to access a database on that machine. MySQL encrypts passwords using a different algorithm than the one used during the Unix login process.
Note that even if the password is stored ‘scrambled’, and knowing your ‘scrambled’ password is enough to be able to connect to the MySQL server!
84) MySQL – When Privilege Changes Take Effect
When mysqld starts, all grant table contents are read into memory and become effective at that point.
Modifications to the grant tables that you perform using GRANT, REVOKE, or SET PASSWORD are noticed by the server immediately.
If you modify the grant tables manually (using INSERT, UPDATE, etc.), you should execute a FLUSH PRIVILEGES statement or run mysqladmin flush-privileges or mysqladmin reload to tell the server to reload the grant tables. Otherwise your changes will have no effect until you restart the server. If you change the grant tables manually but forget to reload the privileges, you will be wondering why your changes don’t seem to make any difference!
When the server notices that the grant tables have been changed, existing client connections are affected as follows:
Table and column privilege changes take effect with the client’s next request.
Database privilege changes take effect at the next USE db_name command.
Global privilege changes and password changes take effect the next time the client connects.
85) How do I rotate replication logs?
In Version 3.23.28 you should use PURGE MASTER LOGS TO command after determining which logs can be deleted, and optionally backing them up first. In earlier versions the process is much more painful, and cannot be safely done without stopping all the slaves in the case that you plan to re-use log names. You will need to stop the slave threads, edit the binary log index file, delete all the old logs, restart the master, start slave threads,and then remove the old log files.
86) How do I upgrade on a hot replication setup?
If you are upgrading pre-3.23.26 versions, you should just lock the master tables, let the slave catch up, then run FLUSH MASTER on the master, and FLUSH SLAVE on the slave to reset the logs, then restart new versions of the master and the slave. Note that the slave can stay down for some time – since the master is logging all the updates, the slave will be able to catch up once it is up and can connect.
After 3.23.26, we have locked the replication protocol for modifications, so you can upgrade masters and slave on the fly to a newer 3.23 version and you can have different versions of MySQL running on the slave and the master, as long as they are both newer than 3.23.26.
87) How can I use replication to improve performance of my system?
You should set up one server as the master, and direct all writes to it, and configure as many slaves as you have the money and rackspace for, distributing the reads among the master and the slaves. You can also start the slaves with –skip-bdb, –low-priority-updates and –delay-key-write-for-all-tables to get speed improvements for the slave. In this case the slave will use non-transactional MyISAM tables instead of BDB tables to get more speed.
88) MySQL – Drawbacks to Creating Large Numbers of Tables in the Same Database
If you have many files in a directory, open, close, and create operations will be slow. If you execute SELECT statements on many different tables, there will be a little overhead when the table cache is full, because for every table that has to be opened, another must be closed. You can reduce this overhead by making the table cache larger.
89) MySQL – Why So Many Open tables?
When you run mysqladmin status, you’ll see something like this:
Uptime: 426 Running threads: 1 Questions: 11082 Reloads: 1 Open tables: 12
This can be somewhat perplexing if you only have 6 tables.
MySQL is multithreaded, so it may have many queries on the same table simultaneously. To minimize the problem with two threads having different states on the same file, the table is opened independently by each concurrent thread. This takes some memory and one extra file descriptor for the data file. The index file descriptor is shared between all threads.
90) How MySQL Optimizes DISTINCT ?
DISTINCT is converted to a GROUP BY on all columns, DISTINCT combined with ORDER BY will in many cases also need a temporary table.
When combining LIMIT # with DISTINCT, MySQL will stop as soon as it finds # unique rows.
If you don’t use columns from all used tables, MySQL will stop the scanning of the not used tables as soon as it has found the first match.
SELECT DISTINCT t1.a FROM t1,t2 where t1.a=t2.a;
In the case, assuming t1 is used before t2 (check with EXPLAIN), then MySQL will stop reading from t2 (for that particular row in t1) when the first row in t2 is found.
91) How MySQL Optimizes LIMIT ?
In some cases MySQL will handle the query differently when you are using LIMIT # and not using HAVING:
If you are selecting only a few rows with LIMIT, MySQL will use indexes in some cases when it normally would prefer to do a full table scan.
If you use LIMIT # with ORDER BY, MySQL will end the sorting as soon as it has found the first # lines instead of sorting the whole table.
When combining LIMIT # with DISTINCT, MySQL will stop as soon as it finds # unique rows.
In some cases a GROUP BY can be resolved by reading the key in order (or do a sort on the key) and then calculate summaries until the key value changes. In this case LIMIT # will not calculate any unnecessary GROUP BY’s.
As soon as MySQL has sent the first # rows to the client, it will abort the query.
LIMIT 0 will always quickly return an empty set. This is useful to check the query and to get the column types of the result columns.
The size of temporary tables uses the LIMIT # to calculate how much space is needed to resolve the query.
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