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

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 = 1SELECT @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     2CALL prc_test();var2  @var2---   ---2     3CALL 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.)