MySQL Interview Questions with Answers

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. ...

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.

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.

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;

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.

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.

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)

How would you select all the users, whose phone number is null?
SELECT user_name FROM users WHERE ISNULL(user_phonenumber);

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

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.

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.

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.

When would you use ORDER BY in DELETE statement?
When you’re not deleting by row ID. Such as in DELETE FROM techpreparation_com_questions ORDER BY timestamp LIMIT 1. This will delete the most recently posted question in the table techpreparation_com_questions.

How can you see all indexes defined for a table?
SHOW INDEX FROM techpreparation_questions;

How would you change a column from VARCHAR(10) to VARCHAR(50)?
ALTER TABLE techpreparation_questions CHANGE techpreparation_content techpreparation_CONTENT VARCHAR(50).

How would you delete a column?
ALTER TABLE techpreparation_answers DROP answer_user_id.

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. ...

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.

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.

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;

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.

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.

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)

How would you select all the users, whose phone number is null?
SELECT user_name FROM users WHERE ISNULL(user_phonenumber);

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

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.

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.

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.

When would you use ORDER BY in DELETE statement?
When you’re not deleting by row ID. Such as in DELETE FROM techpreparation_com_questions ORDER BY timestamp LIMIT 1. This will delete the most recently posted question in the table techpreparation_com_questions.

How can you see all indexes defined for a table?
SHOW INDEX FROM techpreparation_questions;

How would you change a column from VARCHAR(10) to VARCHAR(50)?
ALTER TABLE techpreparation_questions CHANGE techpreparation_content techpreparation_CONTENT VARCHAR(50).

How would you delete a column?
ALTER TABLE techpreparation_answers DROP answer_user_id.


How are ENUMs and SETs represented internally?
As unique integers representing the powers of two, due to storage optimizations.

How do you start and stop MySQL on Windows?
net start MySQL, net stop MySQL

How do you start MySQL on Linux?
/etc/init.d/mysql start

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

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.

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.

How do you change a password for an existing user via mysqladmin?
mysqladmin -u root -p password "newpassword"

Use mysqldump to create a copy of the database?
mysqldump -h mysqlhost -u username -p mydatabasename > dbdump.sql

Have you ever used MySQL Administrator and MySQL Query Browser?
Describe the tasks you accomplished with these tools.

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.

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.

What does myisamchk do?
It compressed the MyISAM tables, which reduces their disk usage.

Explain advantages of InnoDB over MyISAM?
Row-level locking, transactions, foreign key constraints and crash recovery.

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.

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.

How do you control the max size of a HEAP table?
MySQL config variable max_heap_table_size.

What are CSV tables?
Those are the special tables, data for which is saved into comma-separated values files. They cannot be indexed.

Explain federated tables. ?
Introduced in MySQL 5.0, federated tables allow access to the tables located on other databases on other servers.

What is SERIAL data type in MySQL?
BIGINT NOT NULL PRIMARY KEY AUTO_INCREMENT

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.

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.

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.

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.

What happens if a table has one column defined as TIMESTAMP?
That field gets the current timestamp whenever the row gets altered.

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.

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.

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.

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.

General Information About MySQL
MySQL is a very fast, multi-threaded, multi-user, and robust SQL (Structured Query Language) database server.

MySQL is free software. It is licensed with the GNU GENERAL PUBLIC LICENSE http://www.gnu.org/.

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.

ySQL 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.

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.

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.

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.

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.

The Main Features of MySQL
The following list describes some of the important characteristics of MySQL:

Fully multi-threaded using kernel threads. That means it can easily use multiple CPUs if available.
C, C++, Eiffel, Java, Perl, PHP, Python and Tcl APIs.
Works on many different platforms.
Many column types: signed/unsigned integers 1, 2, 3, 4, and 8 bytes long, FLOAT, DOUBLE, CHAR, VARCHAR, TEXT, BLOB, DATE, TIME, DATETIME, TIMESTAMP, YEAR, SET, and ENUM types.
Very fast joins using an optimized one-sweep multi-join.
Full operator and function support in the SELECT and WHERE parts of queries. Example:
mysql> SELECT CONCAT(first_name, " ", last_name) FROM tbl_name
WHERE income/dependents > 10000 AND age > 30;

SQL functions are implemented through a highly optimized class library and should be as fast as they can get! Usually there shouldn't be any memory allocation at all after query initialization.
Full support for SQL GROUP BY and ORDER BY clauses. Support for group functions (COUNT(), COUNT(DISTINCT), AVG(), STD(), SUM(), MAX() and MIN()).
Support for LEFT OUTER JOIN and RIGHT OUTER JOIN with ANSI SQL and ODBC syntax.
You can mix tables from different databases in the same query (as of Version 3.22).
A privilege and password system that is very flexible and secure and allows host-based verification. Passwords are secure because all password traffic is encrypted when you connect to a server.
ODBC (Open-DataBase-Connectivity) support for Win32 (with source). All ODBC 2.5 functions and many others. For example, you can use MS Access to connect to your MySQL server. See section 18 MySQL ODBC Support.
Very fast B-tree disk tables with index compression.
Up to 32 indexes per table are allowed. Each index may consist of 1 to 16 columns or parts of columns. The maximum index length is 500 bytes (this may be changed when compiling MySQL). An index may use a prefix of a CHAR or VARCHAR field. Fixed-length and variable-length records.
In-memory hash tables which are used as temporary tables.
Handles large databases. We are using MySQL with some databases that contain 50,000,000 records and we know of users that uses MySQL with 60,000 tables and about 5,000,000,000 rows
All columns have default values. You can use INSERT to insert a subset of a table's columns; those columns that are not explicitly given values are set to their default values.
Uses GNU Automake, Autoconf, and libtool for portability.
Written in C and C++. Tested with a broad range of different compilers.
A very fast thread-based memory allocation system.
No memory leaks. Tested with a commercial memory leakage detector (purify).
Includes myisamchk, a very fast utility for table checking, optimization, and repair. See section 15 Maintaining a MySQL Installation.
Full support for several different character sets, including ISO-8859-1 (Latin1), big5, ujis, and more. For example, the Scandinavian characters `@ringaccent{a}', `@"a' and `@"o' are allowed in table and column names.
All data are saved in the chosen character set. All comparisons for normal string columns are case insensitive.
Sorting is done according to the chosen character set (the Swedish way by default). It is possible to change this when the MySQL server is started up. To see an example of very advanced sorting, look at the Czech sorting code. MySQL supports many different character sets that can be specified at compile and run time.
Aliases on tables and columns are allowed as in the SQL92 standard.
DELETE, INSERT, REPLACE, and UPDATE return how many rows were changed (affected). It is possible to return the number of rows matched instead by setting a flag when connecting to the server.
Function names do not clash with table or column names. For example, ABS is a valid column name. The only restriction is that for a function call, no spaces are allowed between the function name and the `(' that follows it. See section 7.39 Is MySQL Picky About Reserved Words?.
All MySQL programs can be invoked with the --help or -? options to obtain online assistance.
The server can provide error messages to clients in many languages. See section 10.1 What Languages Are Supported by MySQL?. Clients may connect to the MySQL server using TCP/IP Sockets, Unix Sockets (Unixes), or Named Pipes (NT). The MySQL-specific SHOW command can be used to retrieve information about databases, tables, and indexes. The EXPLAIN command can be used to determine how the optimizer resolves a query.

atabase 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.

Database
A repository to store data.

Table
The part of a database that stores the data. A table has columns or attributes, and the data stored in rows.

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.

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.

Relational model
A model that uses tables to store data and manage the relationship between tables.

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.

SQL
A query language that interacts with a DBMS. SQL is a set of statements to manage databases, tables, and data.

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.

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.

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.

Entity-relationship modeling
A technique used to describe the real-world data in terms of entities, attributes, and relationships.

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.

MySQL Command Interpreter
The MySQL command interpreter is commonly used to create databases and tables in web database applications and to test queries. Throughout the remainder of this chapter we discuss the SQL statements for managing a database. All these statements can be directly entered into the command interpreter and executed. The statements can also be included in server-side PHP scripts, as discussed in later chapters.

Once the MySQL DBMS server is running, the command interpreter can be used. The command interpreter can be run using the following command from the shell, assuming you've created a user hugh with a password shhh:

% /usr/local/bin/mysql -uhugh -pshhh The shell prompt is represented here as a percentage character, %.

Running the command interpreter displays the output:

Welcome to the MySQL monitor. Commands end with ; or \g. Your MySQL connection id is 36 to server version: 3.22.38

Type 'help' for help.

mysql>
The command interpreter displays a mysql> prompt and, after executing any command or statement, it redisplays the prompt. For example, you might issue the statement:

mysql> SELECT NOW( );
This statement reports the time and date by producing the following output:

+---------------------+
| NOW( ) |
+---------------------+
| 2002-01-01 13:48:07 |
+---------------------+
1 row in set (0.00 sec)

mysql>
After running a statement, the interpreter redisplays the mysql> prompt. We discuss the SELECT statement later in this chapter.

As with all other SQL statements, the SELECT statement ends in a semicolon. Almost all SQL command interpreters permit any amount of whitespace—spaces, tabs, or carriage returns—in SQL statements, and they check syntax and execute statements only after encountering a semicolon that is followed by a press of the Enter key. We have used uppercase for the SQL statements throughout this book. However, any mix of upper- and lowercase is equivalent.

On startup, the command interpreter encourages the use of the help command. Typing help produces a list of commands that are native to the MySQL interpreter and that aren't part of SQL. All non-SQL commands can be entered without the terminating semicolon, but the semicolon can be included without causing an error.

The MySQL command interpreter allows flexible entry of commands and SQL statements:

The up and down arrow keys allow previously entered commands and statements to be browsed and used.

The interpreter has command completion. If you type the first few characters of a string that has previously been entered and press the Tab key, the interpreter automatically completes the command. For example, if wines is typed and the Tab key pressed, the command interpreter outputs winestore, assuming the word winestore has been previously used.

If there's more than one option that begins with the characters entered, or you wish the strings that match the characters to be displayed, press the Tab key twice to show all matches. You can then enter additional characters to remove any ambiguity and press the Tab key again for command completion.

Several common statements and commands are pre-stored, including most of the SQL keywords discussed in this chapter.

To use the default text editor to create SQL statements, enter the command edit in the interpreter. This invokes the editor defined by the EDITOR shell environment variable. When the editor is exited, the MySQL command interpreter reads, parses, and runs the file created in the editor.

When the interpreter is quit and run again later, the history of commands and statements is kept. It is still possible to scroll up using the up arrow and to execute commands and statements that were entered earlier.

You can run commands and SQL statements without actually launching the MySQL command interpreter. For example, to run SELECT now( ) from the Linux shell, enter the following command:

mysql -ppassword -e "SELECT now( );" This is particularly useful for adding SQL commands to shell or other scripts.

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