SQL Server遍历表的几种方法

USE TSQLFundamentals2008;
GO

SELECT * FROM HR.Employees;

-- 需求是，新增一列fullname，取值firstname+lastname
ALTER TABLE HR.Employees ADD fullname NVARCHAR(30) NULL;
GO

-- 方法1：游标
-- 声明变量
DECLARE
    @empid AS INT,
    @firstname AS NVARCHAR(10),
    @lastname AS NVARCHAR(20);
    
-- 声明游标
DECLARE C_Employees CURSOR FAST_FORWARD FOR
    SELECT empid,firstname,lastname 
    FROM HR.Employees
    ORDER BY empid;
    
OPEN C_Employees;

-- 取第一条记录
FETCH NEXT FROM C_Employees INTO @empid,@firstname,@lastname;

WHILE @@FETCH_STATUS=0
BEGIN
    -- 操作
    UPDATE HR.Employees SET fullname= @firstname+‘ ‘+@lastname WHERE empid=@empid;
    
    -- 取下一条记录
    FETCH NEXT FROM C_Employees INTO @empid,@firstname,@lastname;
END

-- 关闭游标
CLOSE C_Employees;

-- 释放游标
DEALLOCATE C_Employees;

-- 方法2：使用表变量
-- 声明表变量
DECLARE @temp TABLE
(
    empid INT,
    firstname NVARCHAR(10),
    lastname NVARCHAR(20)
);

-- 将源表中的数据插入到表变量中
INSERT INTO @temp(empid, firstname, lastname )
SELECT empid,firstname,lastname FROM HR.Employees
ORDER BY empid;

-- 声明变量
DECLARE
    @empid AS INT,
    @firstname AS NVARCHAR(10),
    @lastname AS NVARCHAR(20);
    
WHILE EXISTS(SELECT empid FROM @temp)
BEGIN
    -- 也可以使用top 1
    SET ROWCOUNT 1
    SELECT @empid= empid, @firstname= firstname,@lastname= lastname FROM @temp;
    UPDATE HR.Employees SET fullname= @firstname+‘ ‘+@lastname WHERE empid=@empid;
    SET ROWCOUNT 0
    
    DELETE FROM @temp WHERE empid=@empid;
END

-- 方法3：使用临时表
-- 创建临时表
IF OBJECT_ID(‘tempdb.dbo.#tempemployees‘,‘U‘) IS NOT NULL DROP TABLE dbo.#tempemployees;
GO

SELECT empid,firstname,lastname 
INTO dbo.#tempemployees
FROM HR.Employees
ORDER BY empid;

--SELECT * FROM dbo.#tempemployees;

-- 声明变量
DECLARE
    @empid AS INT,
    @firstname AS NVARCHAR(10),
    @lastname AS NVARCHAR(20);
    
WHILE EXISTS(SELECT empid FROM dbo.#tempemployees)
BEGIN
    -- 也可以使用top 1
    SET ROWCOUNT 1
    SELECT @empid= empid, @firstname= firstname,@lastname= lastname FROM dbo.#tempemployees;
    UPDATE HR.Employees SET fullname= @firstname+‘ ‘+@lastname WHERE empid=@empid;
    SET ROWCOUNT 0
    
    DELETE FROM dbo.#tempemployees WHERE empid=@empid;
END

--UPDATE HR.Employees SET fullname=NULL;
--SELECT * FROM HR.Employees;

--USE TSQLFundamentals2008;
--GO

--UPDATE HR.Employees SET fullname= firstname+‘ ‘+lastname;