MySQL allows users to define and use variables within SQL statements. These user-defined variables provide a way to store and manipulate data temporarily during the execution of a series of SQL statements. User-defined variables in MySQL are prefixed with the ‘@’ symbol.

Here’s a basic overview of MySQL user-defined variables:

Variable Declaration

To declare a user-defined variable, use the ‘@’ symbol followed by the variable name. For example:

SET @my_variable = 10;

Variable Scope

User-defined variables have session scope, which means they are visible and accessible only within the session where they are defined. If you declare a variable in one session, it won’t be available in another.

Assigning Values

You can assign values to variables using the SET statement. Variables can store various data types, such as integers, decimals, strings, etc.

SET @name = 'John';
SET @age = 25;
SET @price = 49.99;

Using Variables in Queries

Variables can be used in SQL queries to simplify and parameterize statements.

SELECT * 
FROM users 
WHERE age > @age;

Arithmetic Operations
Variables can participate in arithmetic operations, making it convenient to perform calculations within queries.

SET @total = @price * @quantity;

Updating Variables

Variables can be updated with new values during the execution of the session.

SET @counter = @counter + 1;

Conditional Logic

Variables can be used in conditional statements to control the flow of queries.

IF @age > 18 THEN
    SELECT 'Adult';
ELSE
    SELECT 'Minor';
END IF;

Session Persistence

User-defined variables persist throughout the session but are reset once the session ends. If you need to retain values between sessions, you might consider using session or global variables.

Limitations

While user-defined variables offer flexibility, it’s essential to be cautious about their use. Avoid using them excessively, as they can make code less readable and harder to maintain. Additionally, they may not perform as well as other constructs like stored procedures for complex logic.

In conclusion, MySQL user-defined variables are a powerful feature that allows developers to store and manipulate data temporarily within a session. When used judiciously, they can enhance the readability and flexibility of SQL code. However, it’s crucial to be mindful of their scope and usage to ensure efficient and maintainable code.