Merge Into 用法

从一个数据库的一张表同步数据到另外一个数据库的一张表，同步的数据不是insert就是update。

一般做法是先判断当前数据在另外一张表存不存在，存在则更新，不存在则插入。需要一次查询判断:exist或count(*)>0,

然而用Merge Into 更为简洁：

CREATE PROC [dbo].[XXX_SynchroMaterial]
@userID nvarchar(50)
as
begin
declare @nResult int; --同步多少条物料

declare @totalRows    int;
set @nResult=0;

--建立临时数据表
create table #tmpMateriel
(
[id] int IDENTITY(1,1) ,
MaterielGUID nvarchar(50),
MaterielCode nvarchar(50),
MaterielName  nvarchar(255),
ParentNode nvarchar(50),
IfLeafNode bit,
Material_Category int null,
Raw_Material_Category int null
)  

--读取物料中间表数据，插入到临时表
insert into #tmpMateriel(MaterielGUID,MaterielCode,MaterielName,ParentNode,IfLeafNode,Material_Category,Raw_Material_Category)
SELECT [MaterielGUID]
      ,[MaterielCode]
      ,[MaterielName]
      ,[ParentNode]
      ,[IfLeafNode]
      ,CASE WHEN (MaterielCode like ‘1.03%‘ or MaterielCode like ‘2.02%‘)  THEN 1
           WHEN (MaterielCode like ‘1.04%‘ or MaterielCode like ‘2.03%‘) THEN 2
           ELSE 3
       END
      ,CASE WHEN [IfLeafNode]=1 THEN 4  --小料
       ELSE null
       END

  FROM [ChwareXXXInterface].[dbo].[Tab_Materiel_Interface] where [If_Read]=0;

select @nResult=COUNT(*) from #tmpMateriel;  

Merge Into [ChwareXXX].[dbo].[Tab_Material] t
USING (select [MaterielCode],[MaterielName],[ParentNode],[IfLeafNode],[Material_Category],[Raw_Material_Category] from #tmpMateriel)as s
on t.[MaterialCode]=s.[MaterielCode]
When Matched Then Update set t.[MaterialName]=s.[MaterielName],
                             t.[ParentNode]=s.[ParentNode],
                             t.[IfLeafNode]=s.[IfLeafNode],
                             t.[Material_Category]=s.[Material_Category],
                             t.[Raw_Material_Category]=s.[Raw_Material_Category],
                             t.[ModifyUser]=@userID,
                             t.[ModifyDT]=getdate()
When Not Matched Then Insert ([MaterialCode],[MaterialName],[ParentNode],[IfLeafNode],[Material_Category],[Raw_Material_Category],[CreateUser],[CreateDt])
                      values (s.[MaterielCode],s.[MaterielName],s.[ParentNode],s.[IfLeafNode],s.[Material_Category],s.[Raw_Material_Category],@userID,getdate());

--更新接口表读取状态
update [ChwareXXXInterface].[dbo].[Tab_Materiel_Interface] set [If_Read]=1,ReadDT=GETDATE()where MaterielCode in (select MaterielCode from #tmpMateriel);

--删除临时表
drop table #tmpMateriel;
select @nResult; --同步成功多少条数据
end
GO

总结：

通过MERGE语句，根据一张表或子查询的连接条件对另外一张表进行查询，
连接条件匹配上的进行UPDATE，无法匹配的执行INSERT。
这个语法仅需要一次全表扫描就完成了全部工作，执行效率要高于INSERT＋UPDATE。

语法：
MERGE [INTO [schema .] table [t_alias] 
USING [schema .] { table | view | subquery } [t_alias] 
ON ( condition ) 
WHEN MATCHED THEN merge_update_clause 
WHEN NOT MATCHED THEN merge_insert_clause;