SQL Server 2008对T-SQL语言的增强
来源: 数据库 SQL Server | 作者: landluo | 发布: 2009-7-08 10:09
MERGE 的语法为：
[ WITH [,...n] ]
MERGE
[ TOP ( expression ) [ PERCENT ] ]
[ INTO ] target_table [ [ AS ] table_alias ]
[ WITH ( ) ]
USING
ON
[ WHEN MATCHED [ AND ]
THEN ]
[ WHEN [TARGET] NOT MATCHED [ AND ]
THEN ]
[ WHEN SOURCE NOT MATCHED [ AND ]
THEN ]
[ OPTION ( [ ,...n ] ) ]
;
::=
{ [
[ ,...n ] ]
[ [ , ] INDEX ( index_val [ ,...n ] ) ] }
::=
{
table_or_view_name [ [ AS ] table_alias ] [
]
[ WITH ( table_hint [ [ , ]...n ] ) ]
| rowset_function [ [ AS ] table_alias ]
[ ( bulk_column_alias [ ,...n ] ) ]
| user_defined_function [ [ AS ] table_alias ]
| OPENXML
| derived_table [ AS ] table_alias [ ( column_alias [ ,...n ] ) ]
|
|
|
}
::=
{ UPDATE SET | DELETE }
::=
{ column_name = { expression | DEFAULT | NULL }
| { udt_column_name. { { property_name = expression
| field_name = expression }
| method_name ( argument [ ,...n ] ) } }
| column_name { .WRITE ( expression , @Offset , @Length ) }
| @variable = expression
} [ ,...n ]
::=
INSERT [ ( ) ]
{ VALUES ( )
| DEFAULT VALUES }
::=
{
[ OUTPUT INTO { @table_variable | output_table }
[ (column_list) ] ]
[ OUTPUT ]
}
::=
{ | scalar_expression } [ [AS] column_alias_identifier ]
[ ,...n ]
::=
{ DELETED | INSERTED | from_table_name } . { * | column_name }
| $ACTION
示例：在一条 SQL 语句中使用 WHERE 在一张表上执行 UPDATE 和 DELETE 操作
USE AdventureWorks;
GO
MERGE Production.ProductInventory AS pi
USING (SELECT ProductID, SUM(OrderQty) FROM Sales.SalesOrderDetail sod
JOIN Sales.SalesOrderHeader soh
ON sod.SalesOrderID = soh.SalesOrderID
AND soh.OrderDate = GETDATE()
GROUP BY ProductID) AS src (ProductID, OrderQty)
ON (pi.ProductID = src.ProductID)
WHEN MATCHED AND pi.Quantity - src.OrderQty 0
THEN UPDATE SET pi.Quantity = pi.Quantity - src.OrderQty
WHEN MATCHED AND pi.Quantity - src.OrderQty = 0
THEN DELETE;
这个示例是一个非常典型的销售定货库存问题。这个示例很简单，表达的意思就是：如果某一个产品产生了销售定单数据，则将其对应的产品库存除去该销售定单所产生的数量，如果当前库存数量与该销售定单数量相同，则从库存表中删除该产品的库存纪录。
我们看到，利用 MERGE 语句可以将复杂的 SQL 语句简化。它比起 IF、CASE 等更加灵活和强大。
结论
Microsoft SQL Server 2008 对事务性 SQL 语言做了一些增强，提高了查询效率。使得 SQL Server 成为大中型企业数据库的首先产品。SQL Server 2008 将伴随 Visual Studio 2008 一起发布，开发人员提前了解这些信息有助于在 SQL Server 的新版本发布后快速建立基于该版本的企业级应用程序。
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