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

MySQL: @variable vs. variable. Whats the difference?
 
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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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445down 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.)
 
転載先:https://www.cnblogs.com/kungfupanda/p/5855158.html