问题描述
在我发布的另一个问题中，有人告诉我两者之间存在差异:
@variable和:
变量在 MySQL 中.他还提到了 MSSQL 如何具有批处理范围而 MySQL 具有会话范围.有人可以为我详细说明一下吗?
MySQL 有一个概念 用户定义的变量.
它们是松散类型的变量，可以在会话中的某处初始化并保持它们的值直到会话结束.
它们以 @ 符号开头，如下所示:@var
您可以使用 SET 语句或在查询中初始化此变量:
SET @var = 1选择@var2 := 2在MySQL中开发存储过程时，可以传递输入参数并声明局部变量:
DELIMITER//创建程序 prc_test (var INT)开始声明 var2 INT;设置 var2 = 1;选择 var2;结尾;//分隔符；这些变量前面没有任何前缀.
过程变量和特定于会话的用户定义变量之间的区别在于，每次调用过程时，过程变量都会重新初始化为NULL，而特定于会话的变量则不是:
CREATE PROCEDURE prc_test()开始声明 var2 INT DEFAULT 1;SET var2 = var2 + 1;设置@var2 = @var2 + 1;选择 var2，@var2；结尾;设置@var2 = 1;调用 prc_test();var2 @var2--- ---2 2调用 prc_test();var2 @var2--- ---2 3调用 prc_test();var2 @var2--- ---2 4如您所见，每次调用过程时都会重新初始化var2(过程变量)，而@var2(会话特定变量)则不会.
(除了用户自定义的变量，MySQL还有一些预定义的系统变量"，可能是全局变量"，比如@@global.portcode> 或会话变量"，例如 @@session.sql_mode；这些会话变量"与会话特定的用户定义变量无关.)
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?
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.)
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