MySQL variables are fundamental components of the MySQL database management system (DBMS) that serve as containers for storing temporary values during database operations. The MySQL variables are used to store and manage values temporarily during the execution of queries and scripts. These variables can be classified into two main types: user-defined variables and system variables.

User-defined variables

User-defined variables are created and managed by the user within a session. They can be used to store intermediate results, parameters, or any other values needed during the execution of queries. User-defined variables are prefixed with the ‘@’ symbol.

Syntax:

SET @variable_name = value;

Example:

SET @my_variable = 10;
SELECT @my_variable;

User-defined variables are session-specific, meaning they persist within the current session and are discarded when the session ends.

System variables

System variables are predefined by MySQL and control various aspects of the database server’s behavior. They are used to configure server settings, such as buffer sizes, timeouts, and other operational parameters. System variables are accessible using the @@ syntax.

Syntax:

SELECT @@variable_name;

Example:

SELECT @@max_connections;

System variables have global or session scope, depending on their nature. Global variables affect the entire server, while session variables affect only the current session.

Session variables

Some system variables can be dynamically changed for a specific session using the SET statement. These changes apply only to the current session and do not affect other sessions.

Syntax:

SET GLOBAL variable_name = value; -- For global scope
SET SESSION variable_name = value; -- For session scope

Example:

SET SESSION sql_mode = 'STRICT_ALL_TABLES';

This example sets the SQL mode for the current session, ensuring strict enforcement of table constraints.

Local variables in Stored Procedures

MySQL also allows the use of local variables within stored procedures. These variables are declared using the DECLARE statement and are only accessible within the scope of the stored procedure.

Syntax:

DECLARE variable_name datatype;

Example:

DELIMITER //
CREATE PROCEDURE exampleProcedure()
BEGIN
  DECLARE my_variable INT;
  SET my_variable = 42;
  -- Further logic using my_variable
END //
DELIMITER ;

In this example, my_variable is a local variable within the scope of the exampleProcedure stored procedure.

More Examples of Variable Usage

Calculating Average:

SET @avg_score = (SELECT AVG(score) FROM students);
SELECT * FROM students WHERE score > @avg_score;

Filtering Records Based on a Variable:

SET @order_status = 'pending';
SELECT * FROM orders WHERE status = @order_status;

Dynamically Changing Query Parameters:

SET @min_price = 10;
SET @max_price = 100;
SELECT * FROM products 
WHERE price BETWEEN @min_price AND @max_price;

Benefits of Using Variables

The use of variables in MySQL offers several advantages:

Reusability and Flexibility: Variables allow you to reuse values throughout your queries, reducing redundancy and enhancing code reusability.

Simplified Computations: Variables can store intermediate results, making it easier to perform complex calculations and manipulate data within a single query.

Efficient Query Performance: Variables can improve query performance by reducing the need to repeatedly execute the same subqueries.

Improved Code Readability: Variables can make code more readable and easier to understand by making the logic more self-explanatory.

In summary, MySQL variables, both user-defined and system variables, play a crucial role in managing data, controlling server behavior, and facilitating the development of stored procedures with local variables. Understanding and appropriately using variables contribute to efficient and flexible database interactions.