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

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?
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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.)