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

今天使用Merge语句时遇到了一个问题，当源表数据集为空时，merge就不工作了。下面举一个例子来具体说明一下：

有两个表SourceTable为登陆表，TargetTable为授权表，如果访问用户在登陆表中，更新授权表的授权级别，否则授权0表示游客。sql语句如下：

--SourceTable为登陆表，TargetTable为授权表

--如果访问用户在登陆表中，更新授权表的授权级别，否则授权0表示游客。

create table SourceTable(UserName nvarchar(10),Pwd int,[Level] int)

create table TargetTable(UserName nvarchar(10),[Level] int)

go

--下面是SQL Server2008新增的赋值功能，更多新功能请参见

--http://blog.csdn.net/tjvictor/archive/2009/07/14/4346916.aspx

insert into SourceTable values('user1','111',1),('user2','222',2),('user3','333',3)

insert into TargetTable values('user1',1),('user2',2)

<!-- .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } -->

一般情况，我们会写如下的语句：

declare @UserName varchar(10) = 'user2'--表示某个登陆的用户名

merge into TargetTable as tg

using (select UserName,[Level] from SourceTable where UserName=@UserName)

    as sr(UserName,[Level]) on tg.UserName=sr.UserName

when matched then

    update set [Level]=sr.[Level]--更新授权级别

when not matched by target then

    insert (UserName,[Level]) values(@UserName,0)--表示游客

output $action;

<!-- .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } -->

执行结果为UPDATE，表示执行了更新操作，也正是我们想要的。
但是如果我们把@UserName赋值为'user6’，希望能在TargetTable中插入一条记录，但是实际执行结果为空，显示什么都没执行。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，不知道这是不是SQL Server的bug。

下面的SQL可以解决上面的问题：

declare @UserName varchar(10) = 'user7'--表示某个登陆的用户名

merge into TargetTable as tg

using (select @UserName)

    as sr(UserName) on tg.UserName=sr.UserName

when matched then

    update set [Level]= (select top 1 [Level] from SourceTable where UserName=@UserName)

        --更新授权级别

when not matched by target then

    insert (UserName,[Level]) values(@UserName,0)--表示游客

output $action;

如需要转载，请注明本文原创自CSDN TJVictor专栏：

http://blog.csdn.net/tjvictor/archive/2009/07/21/4367384.aspx

<!-- .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } -->