您的位置：首页 > Web前端

MySQL: @variable vs. variable. Whats the difference?

2016-09-09 08:05 579 查看

MySQL: @variable vs. variable. Whats the difference?

up vote351down votefavorite
121

In another question I posted someone told me that there is a difference between:

@variable

and:

variable

in MySQL. He also mentioned how MSSQL has batch scope and MySQL has session scope. Can someone elaborate on this for me?

add a comment

up vote445down voteaccepted

MySQL

has the concept of user-defined variables.

They are loosely typed variables that may be initialized somewhere in a session and keep their value until the session ends.

They are prepended with an

sign, like this:

@var

You can initialize this variable with a

SET

statement or inside in a query:

SET @var = 1

SELECT @var2 := 2

When you develop a stored procedure in

MySQL

, you can pass the input parameters and declare the local variables:

DELIMITER //

CREATE PROCEDURE prc_test (var INT)
BEGIN
DECLARE  var2 INT;
SET var2 = 1;
SELECT  var2;
END;
//

DELIMITER ;

These variables are not prepended with any prefixes.

The difference between a procedure variable and a session-specific user-defined variable is that procedure variable is reinitialized to

NULL

each time the procedure is called, while the session-specific variable is not:

CREATE PROCEDURE prc_test ()
BEGIN
DECLARE var2 INT DEFAULT 1;
SET var2 := var2 + 1;
SET @var2 := @var2 + 1;
SELECT  var2, @var2;
END;

SET @var2 = 1;

CALL prc_test();

var2  @var2
---   ---
2     2

CALL prc_test();

var2  @var2
---   ---
2     3

CALL prc_test();

var2  @var2
---   ---
2     4

As you can see,

var2

(procedure variable) is reinitialized each time the procedure is called, while

@var2

(session-specific variable) is not.

(In addition to user-defined variables, MySQL also has some predefined "system variables", which may be "global variables" such as

@@global.port

or "session variables" such as

@@session.sql_mode

; these "session variables" are unrelated to session-specific user-defined variables.)

内容来自用户分享和网络整理，不保证内容的准确性，如有侵权内容，可联系管理员处理

标签：

相关文章推荐

新的分享

章节导航