How to use declare in mysql

DECLARE is used to declare variables or cursors in MySQL: declare variables: DECLARE variable_name data_type [DEFAULT value]; declare cursors: DECLARE cursor_name CURSOR FOR query;

Usage of DECLARE in MySQL

DECLARE is used in MySQL to declare variables or cursors so that you can use them in your code.

Syntax

DECLARE variable_name data_type [DEFAULT value];

Parameters

• variable_name:The name of the variable, must follow MySQL identifier naming rules.
• data_type:The data type of the variable, such as INT, VARCHAR.
• DEFAULT value:Optional, set the default value of the variable.

Cursor declaration

DECLARE cursor_name CURSOR FOR query;

Among them,queryis a SELECT statement used to define the query results of the cursor.

Usage

Declaring variables

DECLARE can be used to declare temporary variables, store intermediate values or serve as loop counters. For example:

DECLARE counter INT DEFAULT 0;

Declaring a Cursor

DECLARE can be used to declare a cursor to iterate over a result set in code. For example:

DECLARE cursor_emp CURSOR FOR SELECT * FROM employees;

Variables declared using the DECLARE variable

can be used in subsequent statements just like ordinary variables. For example:

SET counter = counter + 1;

A cursor declared using the DECLARE cursor

can be operated using the following statement:

• FETCH:Retrieve a row of data from the cursor.
• MOVE:Move the cursor to the specified position.
• CLOSE:Close the cursor and release resources.

Example

The following example demonstrates how to use DECLARE to implement a simple counter:

DECLARE counter INT DEFAULT 0; -- 循环 10 次并递增计数器 WHILE counter < 10 DO SET counter = counter + 1; END WHILE; -- 输出计数器的值 SELECT counter;

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