探讨Sql Server中的declare基本知识

DECLARE @variable_name datatype;

-- 声明一个整数变量，还可直接这样定义 DECLARE @MyNumber INT=10;
DECLARE @MyNumber INT;
-- 为变量赋值
SET @MyNumber = 10;
-- 使用变量
SELECT @MyNumber AS 'My Number';

-- 声明变量
DECLARE @FirstName VARCHAR(50);
DECLARE @LastName VARCHAR(50);
DECLARE @FullName VARCHAR(100);
-- 赋值
SET @FirstName = 'John';
SET @LastName = 'Doe';
-- 连接字符串
SET @FullName = @FirstName + ' ' + @LastName;
-- 使用变量
SELECT @FullName AS 'Full Name';

-- 声明变量
DECLARE @Threshold INT;
DECLARE @Result VARCHAR(50);
-- 赋值
SET @Threshold = 50;
-- 条件判断
IF @Threshold > 100
    SET @Result = 'Above 100';
ELSE
    SET @Result = '100 or Below';
-- 使用变量
SELECT @Result AS 'Result';

-- 声明变量
DECLARE @Counter INT;
DECLARE @MaxCount INT;
-- 赋值
SET @Counter = 1;
SET @MaxCount = 5;
-- 循环
WHILE @Counter <= @MaxCount
BEGIN
    PRINT 'Counter is ' + CAST(@Counter AS VARCHAR(10));
    SET @Counter = @Counter + 1;
END

SET @variable_name = value;

-- 声明并赋值变量
SET @MyNumber = 10;
-- 使用变量
SELECT @MyNumber AS 'My Number';

-- 声明并赋值变量
SET @FirstName = 'John';
SET @LastName = 'Doe';
-- 连接字符串
SET @FullName = CONCAT(@FirstName, ' ', @LastName);
-- 使用变量
SELECT @FullName AS 'Full Name';

– 声明并赋值变量
SET @Threshold = 50;
– 条件判断
IF @Threshold > 100 THEN
SET @Result = ‘Above 100';
ELSE
SET @Result = ‘100 or Below';
END IF;
– 使用变量
SELECT @Result AS ‘Result';

DELIMITER $$
CREATE PROCEDURE LoopExample()
BEGIN
    DECLARE Counter INT DEFAULT 1;
    DECLARE MaxCount INT DEFAULT 5;
    -- 使用准备好的语句进行循环
    PREPARE stmt FROM 'SELECT ? AS Counter';
    WHILE Counter <= MaxCount DO
        EXECUTE stmt USING @Counter;
        SET Counter = Counter + 1;
    END WHILE;
    DEALLOCATE PREPARE stmt;
END$$
DELIMITER ;
-- 调用存储过程
CALL LoopExample();

-- 声明并赋值变量
SET @Threshold = 50;
-- 条件判断
IF @Threshold > 100 THEN
    SET @Result = 'Above 100';
ELSE
    SET @Result = '100 or Below';
END IF;
-- 使用变量
SELECT @Result AS 'Result';

-- 声明并赋值变量
SET @Counter = 1;
SET @MaxCount = 5;
-- 使用准备好的语句进行循环
PREPARE stmt FROM 'SELECT ? AS Counter';
WHILE @Counter <= @MaxCount DO
    EXECUTE stmt USING @Counter;
    SET @Counter = @Counter + 1;
END WHILE;
DEALLOCATE PREPARE stmt;