MySQL: @variable vs. variable. What's the difference?

MySQL has a 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 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 a 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.)

In MySQL, @variable indicates a user-defined variable. You can define your own.

SET @a = 'test';
SELECT @a;

Outside of stored programs, a variable, without @, is a system variable, which you cannot define yourself.

The scope of this variable is the entire session. That means that while your connection with the database exists, the variable can still be used.

This is in contrast with MSSQL, where the variable will only be available in the current batch of queries (stored procedure, script, or otherwise). It will not be available in a different batch in the same session.

MSSQL requires that variables within procedures be DECLAREd and folks use the @Variable syntax (DECLARE @TEXT VARCHAR(25) = 'text'). Also, MS allows for declares within any block in the procedure, unlike MySQL which requires all the DECLAREs at the top.

While good on the command line, I feel using the set = @variable within stored procedures in MySQL is risky. There is no scope and variables live across scope boundaries. This is similar to variables in JavaScript being declared without the var prefix, which are then the global namespace and create unexpected collisions and overwrites.

I am hoping that the good folks at MySQL will allow DECLARE @Variable at various block levels within a stored procedure. Notice the @ (at sign). The @ sign prefix helps to separate variable names from table column names - as they are often the same. Of course, one can always add an "v" or "l_" prefix, but the @ sign is a handy and succinct way to have the variable name match the column you might be extracting the data from without clobbering it.

MySQL is new to stored procedures and they have done a good job for their first version. It will be a pleasure to see where they take it form here and to watch the server side aspects of the language mature.