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

<code>DECLARE variable_name data_type [DEFAULT value];</code>

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

<code>DECLARE cursor_name CURSOR FOR query;</code>

Among them, query is 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:

<code class="sql">DECLARE counter INT DEFAULT 0;</code>

Declaring a Cursor

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

<code class="sql">DECLARE cursor_emp CURSOR FOR SELECT * FROM employees;</code>

Variables declared using the DECLARE variable

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

<code class="sql">SET counter = counter + 1;</code>

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:

<code class="sql">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;</code>

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