SQL Server 2008 Merge into

--创建测试表  
CREATE TABLE t_A 
(MCLASS VARCHAR(20),SNO VARCHAR(20))  
INSERT INTO t_A  
SELECT 'L-1','123'  
UNION ALL  
SELECT  'L-2','234'  
UNION ALL  
SELECT 'L-3','345'

CREATE TABLE t_B 
(MCLASS VARCHAR(20),SNO VARCHAR(20))  
INSERT INTO t_B  
SELECT 'L-1','999'  
UNION ALL  
SELECT  'L-2','999'  
UNION ALL  
SELECT 'L-4','888'

--确定目标表
merge into t_A as a  
--从t_B表中查找MCLASS相同的数据
using t_B as b on a.MCLASS=b.MCLASS 
--当MCLASS相同时，则更新t_A中SNO字段
when Matched then update set a.SNO=b.SNO 
--当t_A表中不存在MCLASS相同的数据时，则从t_B表插入目标表t_A
when Not Matched then Insert(MCLASS,SNO) values(b.MCLASS,b.SNO);

select * from t_A
select * from t_B

drop table t_A
drop table t_B

t_a

MCLASS               SNO

-------------------- --------------------
L-1                  123
L-2                  234
L-3                  345

(3 行受影响)
t_b
MCLASS               SNO
-------------------- --------------------
L-1                  999
L-2                  999
L-4                  888

(3 行受影响)

----------------------------------------------------------------------

更新后的t_a

MCLASS               SNO
-------------------- --------------------
L-1                  999
L-2                  999
L-3                  345
L-4                  888

(4 行受影响)

Merge语句可以将一个表中的数据合并到另一个表中，在合并的同时可以进行插入、删除、更新等操作。