DJTripleThreat DJTripleThreat - 1 year ago 86
MySQL Question

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

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?

Answer Source

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.)