2008之后用Merge语句,
之前就要自己写if条件了.

Merge:

http://technet.microsoft.com/zh-cn/library/cc879317(v=sql.105).aspx

若要提高 MERGE 语句的性能，我们建议您遵循以下索引准则：

• 对源表的联接列创建唯一的涵盖索引。

• 对目标表的联接列创建唯一的聚集索引。

MERGE dbo.FactBuyingHabits AS Target
USING (SELECT CustomerID, ProductID, PurchaseDate FROM dbo.Purchases) AS Source
ON (Target.ProductID = Source.ProductID AND Target.CustomerID = Source.CustomerID)
WHEN MATCHED THEN
    UPDATE SET Target.LastPurchaseDate = Source.PurchaseDate
WHEN NOT MATCHED BY TARGET THEN
    INSERT (CustomerID, ProductID, LastPurchaseDate)
    VALUES (Source.CustomerID, Source.ProductID, Source.PurchaseDate)
OUTPUT $action, Inserted.*, Deleted.*; 

======================================================

create table t_test(id int primary key,value varchar(50))

insert into T_Test
values(111,'aaa')
go

declare @id int
declare @value varchar(50)

set @id = 111
set @value = '111'

if exists(select 1 from t_test where id=@id)
update T_Test
set value = @value
where ID = @id
else
insert into T_Test
select @id,@value

--插入重复数据后,发现数据被更新了,没有报错
select * from t_test

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 MERGE问题举例：

SQL Server使用Merge语句当源表数据集为空时，无法进行查询的问题：http://www.jb51.net/article/19281.htm

 原因就是using (select UserName,[Level] from SourceTable where UserName=@UserName) as sr(UserName,[Level]) on tg.UserName=sr.UserName这个语句中，sr结果集为空，所以merge语句就不向后执行了。

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附加：MYSQL一句sql解决：http://50vip.com/blog.php?i=347