MySQL Interview Questions and Answers

MySQL is one of the widely used open source database platforms in the world today. The data in a MySQL database is stored in the form of tables, which consist of many columns and rows. It is a reliable, stable and robust platform with advanced performance and security features.

MySQL is used for a wide range of purposes like data warehousing, e-commerce, web applications, and logging applications.

It is developed by David Axmark.

The first version of MySQL was released on 23 May 1995.

It is a language but not a Programming Language. It is a relational database management system.

• Data Security
• It is used to retrieve large amounts of records from a database quickly and efficiently.
• On-Demand Scalability
• High Performance
• Excellent Uptime
• Reduced Total Cost of Ownership

• Mysql is available free because it is open source, but SQL is not open-source.
• Mysql offers updatable views, but SQL offers indexed views that are more powerful.
• Mysql does not support XML, but SQL supports XML.
• Mysql not supported Auto tuning but SQL can.
• Mysql did not support User-defined functions, but SQL supported.
• Mysql support Transaction is very much limited but SQL support extensively and entirely offered.
• Mysql not offered Stored procedures and full joins but offered in SQL.
• Mysql does not support the Cursor feature but is there in SQL.

Storage engines are Mysql components, that can handle the operations for different table types to store and manage information in a database.

• InnoDB
• MyISAM
• MEMORY
• MERGE
• EXAMPLE
• CSV
• ARCHIVE etc

ALTER TABLE EnterTableName ENGINE = EnterEngineName;

InnoDB

These are most commonly used storage engines in MySQL are MyISAM and InnoDB.

Difference between MyISAM and InnoDB

• MyISAM no longer supports transactions, however, InnoDB supports transactions.
• MyISAM helps Table-level Locking, however, InnoDB supports Row-level Locking.
• MyISAM helps full-text search, however InnoDB does not.
• MyISAM is designed for the need for speed but InnoDB is designed for the most performance.
• MyISAM does now not aid overseas keys, but InnoDB helps foreign keys.
• We can use commit and rollback with InnoDB however not in MyISAM.
• MyISAM does no longer assist ACID (Atomicity, Consistency, Isolation, and Durability) however InnoDB supports the ACID property.
• In the InnoDB table, the AUTO_INCREMENT field is a section of the index but is now not in MyISAM.
• MyISAM stores its tables, data, and indexes in the disk area with the usage of a separate table name.FRM, desk name.MYD and table name.MYI however InnoDB stores its tables and indexes in a tablespace.

The default port is 3306. We can change it in /etc/MySQL/my.conf there is a port variable. We can update this port according to our need

Mysql supports various data types. Some most common used data types are given below:-

• CHAR
• VARCHAR
• TINYTEXT
• TEXT
• INT
• FLOAT
• DATE
• DATETIME
• TIMESTAMP
• TIME
• ENUM
• DECIMAL
• TINYINT
• LONGTEXT
• MEDIUMTEXT etc

You can use this query in your PHPMyAdmin SELECT version();

MySQL -u username -p database_name < file.sql
Here username is your database username like "root," database_name is your database name, file.sql is your SQL file name with complete path.

The MySQL aggregate function performs a calculation on a set of values, and it returns a single value as output. It ignores NULL values when it performs calculation except for the COUNT function.

MySQL provides many aggregate functions that are listed below.

• AVG()
• COUNT()
• SUM()
• MIN()
• MAX() etc

In Mysql, joins are used to retrieve data from multiple tables. It is performed whenever two or more tables are joined.

There are three types of MySQL joins:-

• MySQL INNER JOIN
  SELECT columns FROM table_1 INNER JOIN table_2 ON table_1.column = table_2.column;
• MySQL LEFT JOIN
  SELECT columns FROM table_1 LEFT JOIN table_2 ON table_1.column = table_2.column;
• MySQL RIGHT JOIN
  SELECT columns FROM table_1 RIGHT JOIN table_2 ON table_1.column = table_2.column;

In Mysql, a trigger is a database object that is directly associated with a table. It will be activated when a defined action is executed for the table.

It can be performed when you run one of the following MySQL like INSERT, UPDATE and DELETE occurred in a table. Its activation time can be BEFORE or AFTER.

Types of Triggers in MySQL

• Before Insert: This is done before inserting data into the table.
• After Insert: This is after data has been entered into the table.
• Before update: This is activated prior to the updating of the data in the table.
• After update: This function is activated upon the updating of the table's data.
• Before delete: This is done before any data are removed from the table.
• After delete: This function is activated when the data has been deleted from the table.

What is the purpose of triggers in MySQL?

• It is easy to set up triggers.
• Triggers allow us to validate data before it is inserted or updated.
• Triggers allow us to keep track of records, such as audit trails for tables.
• Triggers increase the performance of SQL queries as it doesn't need to compile every time the query executes.
• Triggers can be used to reduce client-side code, which saves time and effort.
• It allows us to scale our applications across multiple platforms.
• SQL triggers are an alternative method to verify the integrity of data.
• Triggers are an alternative method to perform the task.

Syntax

CREATE TRIGGER trigger_name    
    (AFTER | BEFORE) (INSERT | UPDATE | DELETE)  
         ON table_name FOR EACH ROW    
         BEGIN
        --trigger code    // You can write here your code
        END;

• CHAR can have a maximum of 255 characters, but VARCHAR can hold a maximum of 65,535 characters.
• CHAR field is a fixed length, but VARCHAR is a variable length field.
• CHAR uses static memory allocation, but VARCHAR uses dynamic memory allocation.

We can use SELECT NOW();

SELECT * FROM EmployeeTable1 AS Emp1 WHERE (1) = ( SELECT COUNT(DISTINCT(Emp2.amount)) FROM EmployeeTable2 AS Emp2 WHERE Emp2.amount > Emp1.amount)

You can use
CREATE INDEX [index name] ON [table name]([column name]);

Comparisons operators are used to comparing one expression to another value or expression. It is just like = , < , > , => , =<, <>

The stored procedure is like a subprogram in a typical computing language which is stored in the database. A stored procedure contains the name, list of parameters, and the SQL statements. All the relational database system works as pillars for stored procedures.

In this example, we are creating a simple procedure called job_data, when this procedure will get executed, all the data from "jobs" tables will get displayed.

DELIMITER //
CREATE PROCEDURE GetAllPages()
BEGIN
    SELECT * FROM pages WHERE title LIKE '%MySQL Interview Questions%';
END //
DELIMITER ;

SELECT * FROM 'TableName' WHERE 'status' = 1 LIMIT 10

WHERE term is used for filtering rows, and it applies to every row but HAVING term is used to filter groups.

WHERE can be used without the GROUP BY but HAVING clause cannot be used without the GROUP BY.

SELECT DISTINCT name FROM Users;

// Here name is the field & Users is the Table name

SHOW INDEX FROM TABLE_NAME;

It is used to concatenate two or more strings.

select count(*) from TABLE_NAME where 'status' = 1;

IS NULL checks to see if the cell is empty but IS NOT NULL checks to see if the cell is not empty.

Example :

SELECT id FROM users WHERE 'user_type' IS NOT NULL;

SELECT id FROM users WHERE 'user_type' NOT IS NULL;

Note: This is very essential MySQL query interview questions.

It is the best way to upload large CSV files through the LOAD DATA command.

Syntax:

LOAD DATA LOCAL INFILE "your file name" INTO TABLE table name FIELDS TERMINATED by ’,’ LINES TERMINATED BY ’\n’

Between operator is used to select a range of data between two values. It can be texts, numbers, and dates, etc.

Syntax: Select * from TABLENAME where FIELDNAME between VALUE1 and VALUE2

IN operator is used to check for a value in the given set of values.

Syntax: Select * from TABLENAME where FIELDNAME IN ('VALUE1','VALUE2',...)

UPDATE `your_table_name` SET new_field=old_field

To write an optimized query in MySQL, below are some guidelines:

• Functions should not be used in predicates.
• At the beginning of predicates, do not use the wildcard symbols such as %.
• Use only needed columns in the SELECT clause.
• Always use the inner join option.
• Use of the “Order by” clause is necessary for SQL, in case of assuming the sorted results.

In general slow query, the log is basically used in Mysql for the determination of which database queries will take a long duration to run. Moreover, a slow query log in MySQL simplifies that operations in the context of efficient and time-consuming queries.

In order to enable the slow query log the command- Get global slow_query_log – ‘ON’; is used

There are a number of unique features that are incorporated in Mysql 8 include-

• Unicode 9.0 support
• Window functions
• Recursive SQL syntax statements
• Support Native JSON data
• Support for document store Functionality

You can use DESC table_name

SELECT std_id, COUNT(std_id) as cnt FROM Student GROUP by std_id having cnt > 1

SELECT id, name, department FROM Employee where MOD(id,2) = 1

SELECT * FROM Student where MOD(id,2) = 0

SELECT EmpId, Project, Salary, COUNT(*) FROM EmployeeSalary GROUP BY EmpId, Project, Salary HAVING COUNT(*) > 1;

Using INTERSECT

SELECT * FROM EmployeeSalary
INTERSECT
SELECT * FROM ManagerSalary

These are the set of rules applied to columns on the table. It is used to bound the type of data that can go into a table.

Types of constraints

• PRIMARY KEY
• FOREIGN KEY
• UNIQUE:
• Not NULL

Timestamp in MySQL

Timestamp in MySQL is used when users want to convert the value from the current time zone to UTC zone for the sake of storage or UTC time zone to the current time zone for retrieval.

DateTime in MySQL

DateTime in MySQL is used to insert values ‘yyyy-mm-dd’ in the table where yyyy is the year, mm is the month and dd is the date. This keyword is used to retrieve or to display DATETIME values in Mysql.

The view is basically a virtual table which is used to view certain and preferable rows or columns of the table. In practice, users can add a number of SQL functions, JOIN and WHERE clause to view the present data of the table.

In MySQL, the CREATE VIEW statement is used to create a new view inside the database.

Here’s the basic syntax of the statement:

CREATE VIEW productList AS SELECT qty, price, totalprice AS value FROM product;

I order to validate the email addresses in Mysql, below mentaioned query is used –
SELECT * FROM `users` WHERE `email` NOT REGEXP '^[^@]+@[^@]+\.[^@]{2,}$';

Binary data can be stored in Mysql in different ways such as

• The short length of binary data can be stored in the form of string as a VARCHAR
• A BLOB data type allows users to store an arbitrary amount of binary data.

DELETE FROM Admin WHERE id IN(SELECT * FROM(SELECT id FROM Admin GROUP BY name HAVING COUNT(*) > 1) AS A)

Regex is an operational and powerful pattern that can help the users for the implementation of a powerful search utility function for database systems. Moreover, it is an operator which is used when we require matching regular expressions. In addition to this, it also supports a wider range of meta-characters which allow flexibility and more control over the database structure when performing pattern matching.

Mysql is not case sensitive. Its case-sensitivity relies on the underlying operating system as OS determines the case sensitivity of tables names and database. In windows, database and table names are not case sensitive but in the case of UNIX, it is case sensitive in nature. Especially on the UNIX host, database accepts the upper case and lower-case table names.

It is a table that points to a table in another MySQL database instance. It can be seen as a view of this remote database. Other RDBMS have the same concepts for example database links.

The maximum size of the HEAP table can be controlled by the MySQL config variable called max_heap_table_size.

SELECT id, GROUP_CONCAT(department SEPARATOR ' ') AS department FROM employee group by id;

• Kana Sensitivity
• Case Sensitivity
• Accent Sensitivity
• Width Sensitivity etc

HEAP tables are in memory. No TEXT or BLOB fields are allowed inside HEAP tables. They are typically used for high-speed transient storage.

In MySQL, a temporary table is a distinct type of table that approves you to save a transient end result set, in which you can reuse countless instances in a single session. A brief table is created with the aid of the use of the CREATE TEMPORARY TABLE statement.

• Don't use "sp_" while creating a stored procedure.
• Don't use "*" while you are using the SELECT command. Also avoid unnecessary columns in SELECT clause.
• You can use table aliases while writing queries.
• Avoid the usage of the wildcard (%) at the starting of a predicate.
• DISTINCT and UNION need to be used solely if it is necessary.

A database index is the structure of a fact that improves the velocity of operations in a table. It can be created the usage of one or greater columns, presenting the foundation for each fast random lookups and efficient ordering of getting admission to records. Indexes are used to locate rows with specific column values quickly.

In MySQL, the time period cardinality refers to the specialty of facts values that can be put into columns. It is a type of property that influences the potential to search, cluster and kind data.

Cardinality can be of two sorts which are as follows

• Low Cardinality − All values for a column have to be the same.
• High Cardinality − All values for a column ought to be unique.

The degree of a table is the number of columns in the given table. It is also called as Arity. It is the range of entities inside each entity type that can be linked via a given relationship type.

1. DDL

In MySQL, DDL is the short form for Data Definition Language, which is used in database schemas and descriptions while deciding how data should reside in the database.

Here’s a list of DDL Queries:

• CREATE
• ALTER
• DROP
• TRUNCATE
• COMMENT
• RENAME

2. DML

DML is a short form for Data Manipulation Language which is used in data manipulation and mostly includes common SQL statements to store, modify, retrieve, delete and update data in a database.

Here is the list of DML Queries:

• SELECT
• INSERT
• UPDATE
• DELETE
• MERGE
• CALL
• EXPLAIN PLAN
• LOCK TABLE

3. DCL

DCL is a short form for Data Control Language including commands which are concerned with User rights, permissions and other controls within the database system.

Here’s a list of queries for DCL:

• GRANT
• REVOKE

The standard way to perform case insensitive queries in SQL is to use the SQL upper or lower functions like the following:

select * from users where upper(first_name) = 'AJAY';

OR

select * from users where lower(first_name) = 'ajay';

The method is to make the field you are searching as uppercase or lowercase then also make the search string uppercase or lowercase as per the SQL function.

SELECT id, name FROM users WHERE phone IS NULL;

Here’s a code script to convert string data to UTF 8 in your database:

header('Content-Type: text/plain; charset=utf-8');
$databasesToConvert = [ 'database1' */ ];
$typesToConvert = [ 'char', 'varchar', 'tinytext', 'mediumtext', 'text', 'longtext'];
CONVERT(CAST(CONVERT(`{$column}` USING english) AS binary) USING utf8)";
$query = "UPDATE `{$table}` SET " . join( ', ', $converts );
echo "\n", $query, "\n";
echo "\n";
 

In MySQL, standards-based drivers for JDBC, ODBC, and .Net are provided in order to enable developers in building database applications with their language of choice.

Available Drivers in MySQL:

• PHP Driver
• JDBC Driver
• ODBC Driver
• C WRAPPER
• PYTHON Driver
• PERL Driver
• RUBY Driver
• CAP11PHP Driver
• Ado.net5.mxj

JDBC, ODBC, and .Net drivers are necessary for MySQL as they enable the developers for building their database applications.

In MySQL, a trigger corresponds to a set of actions running automatically when a particular change operation like SQL INSERT, UPDATE, or DELETE query is performed on a table.

In MySQL, an ENUM is actually a string object whose value is selected from the list of permitted values pre-defined during the time of a column creation. It is used because it provides compact data storage in addition to readable queries and output.

Here’s the difference between an ENUM and a SET

Yes, the primary key can be dropped in MySQL with the use of the “ALTER TABLE” statement.

Syntax:

ALTER TABLE table_name DROP PRIMARY KEY

Images in MySQL can be stored as blobs. For saving them: All the database images are converted into the blobs first. Then, they will get inserted into the database, and later on, it will get stored into the disk.

SELECT CONCAT(first_name, ' ', middle_name, '', last_name) as name FROM `customers`;

SELECT COUNT(*) FROM customers WHERE created_at BETWEEN '2022-03-24 17:32:05' AND '2022-03-25 17:32:05';

SELECT * FROM customers WHERE name LIKE 'A%';

SELECT name, email, min(salary) as minSalary, max(salary) as maxSalary from employee;

SELECT name, salary FROM Employee AS emp1 WHERE N-1 = (SELECT COUNT(DISTINCT salary) FROM Employee emp2 WHERE emp2.salary > emp1.salary);

SELECT * FROM `customers` WHERE `email` NOT REGEXP '^[^@]+@[^@]+\.[^@]{2,}$';

SELECT id, name, email, salary, IF(salary > 20000, 'Good', 'Average') AS type FROM `customers`;

The following SQL query can be used to obtain unique or different values for a column in MySQL Table. There are different values that can be assigned to one or more columns. You must separate the column names with a comma.

Syntax

SELECT DISTINCT(column_name) FROM your_table_name;

Example

SELECT DISTINCT(email) FROM Employee;

For this, we can use the between operator with a where clause.

SELECT id, name, salary FROM Employee WHERE salary BETWEEN 5000 AND 25000;

To insert new records into a table, use the INSERT INTO statement. You can write the INSERT INTO sentence in one of two ways.

• You will need to specify both column names and values that you wish to insert:
  INSERT INTO table_name (column1, column2, column3, ...) VALUES (value1, value2, value3, ...);
• You don't need to include the column names in your SQL query if you are adding values to all columns in the table. But, ensure that the order of your values matches the order of the columns in the table. The INSERT INTO syntax is as follows:
  INSERT INTO table_name VALUES (value1, value2, value3, ...);

Example

INSERT INTO Customers (name, email, salary) VALUES ('MySQL Query Interview Questions', '[email protected]', 15000);