SQL Server循环——游标、表变量、临时表

 游标

在游标逐行处理过程中，当需要处理的记录数较大，而且游标处理位于数据库事务内时，速度非常慢。

-- 声明变量
DECLARE @Id AS Int
-- 声明游标
DECLARE C_Id CURSOR FAST_FORWARD READ_ONLY FOR
    SELECT  b.Id FROM  dbo.Books b;
-- 打开游标
OPEN C_Id;

-- 取第一条记录
FETCH NEXT FROM C_Id INTO @Id;
WHILE @@FETCH_STATUS = 0
    BEGIN
    --逻辑处理
        SELECT  *  FROM  dbo.Books WHERE Id = @Id;
   -- 取下一条记录
        FETCH NEXT FROM C_Id INTO @Id;
    END;
-- 关闭游标
CLOSE C_Id;
-- 释放游标
DEALLOCATE C_Id;

临时表

-- 创建临时表
IF OBJECT_ID('tempdb.dbo.#tempBooks','U') IS NOT NULL DROP TABLE dbo.#tempBooks;
GO

SELECT Id 
INTO dbo.#tempBooks
FROM  dbo.Books b

DECLARE
  @Id Int
    
WHILE EXISTS(SELECT Id FROM dbo.#tempBooks)
BEGIN
    -- 也可以使用top 1
    SET ROWCOUNT 1

    --逻辑处理
    SELECT @Id=Id FROM dbo.#tempBooks;
    SELECT  *  FROM  dbo.Books WHERE Id = @Id;

    SET ROWCOUNT 0
    -- 删除临时表
    DELETE FROM dbo.#tempBooks WHERE Id=@Id;
END

DECLARE @intMinId INT ,
    @intMaxId INT;

SELECT  RowID = IDENTITY( INT,1,1)--使用identity(int,1,1)来产生行号，必须把数据插入到一张表中。
INTO    #templist
FROM    dbo.Books;

SELECT  @intMinId = MIN(RowID) ,
        @intMaxId = MAX(RowID)
FROM    dbo.#templist;

WHILE @intMinId <= @intMaxId
    BEGIN
        PRINT @intMinId;
        SET @intMinId = @intMinId + 1;
    END;
DROP TABLE dbo.#templist;

 表变量

-- 声明表变量
DECLARE @tempBooks TABLE
(
     Id Int
);

-- 将源表中的数据插入到表变量中
INSERT INTO @tempBooks(Id)
SELECT Id  FROM dbo.Books
-- 声明变量
DECLARE
   @Id Int
    
WHILE EXISTS(SELECT Id FROM @tempBooks)
BEGIN
    -- 也可以使用top 1
    SET ROWCOUNT 1
    
    --逻辑处理
    SELECT @Id = Id FROM @tempBooks;
    SELECT  *  FROM  dbo.Books WHERE Id = @Id;

    SET ROWCOUNT 0

    DELETE FROM @tempBooks WHERE Id = @Id;
END