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

时间：2023-12-18 17:51:44

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?
 
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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.)

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