select employeeid,employeename from employee ----1
-----------------2
with MyCTE(ID, Name)
as
(
select employeeid as ID, employeename as Name
from employee
)
select * from MyCTE
请问大家上述两个问题有什么区别,那个sql会好些?另外顺便能否提供一些row_numer()over(),CTE的相关详解用法,谢谢
-----------------2
with MyCTE(ID, Name)
as
(
select employeeid as ID, employeename as Name
from employee
)
select * from MyCTE
请问大家上述两个问题有什么区别,那个sql会好些?另外顺便能否提供一些row_numer()over(),CTE的相关详解用法,谢谢
如果单单是你现在给的这个例子, 当然第一个好一点, CTE是用来处理复杂一点的东西的, 你这个语句没必要用CTE的,具体可以参照联机丛书CTE的用法.
通用表表达式(CTE)是一个可以由定义语句引用的临时表命名的结果集。在他们的简单形式中,您可以将CTE视为类似于视图和派生表混合功能的改进版本。在查询的FROM子句中引用CTE的方式类似于引用派生表和视图的方式。只须定义CTE一次,即可在查询中多次引用它。在CTE的定义中,可以引用在同一批处理中定义的变量。但是CTE的真正威力在于它们的递归功能,即CTE可以包含对它们自身的引用。视图、派生表和CTE内部的查询的一般形式1、视图
CREATE VIEW <view_name>(<column_aliases>) AS <view_query>2、派生表
SELECT * FROM (<derived_table)query>) AS <dericed_table_alias>(<column_aliases>)3、CTE
WITH <cte_alias>(<column_aliases>)
AS
{
<cte_query>
)
SELECT * FROM <cte_alias]>
在关键字WITH之后,为CTE提供一个别名,并且为它的结果列提供一个可选的别名列表;编写CTE的主体;然后从外部查询中引用它。4、演示代码
USE AdventureWorks
GO
WITH SalesCTE(ProductID, SalesOrderID)
AS
(
SELECT ProductID, COUNT(SalesOrderID)
FROM Sales.SalesOrderDetail
GROUP BY ProductID
)
SELECT * FROM SalesCTE
语法:ROW_NUMBER () OVER ( [ <partition_by_clause> ] <order_by_clause> ) 。
备注:ORDER BY 子句可确定在特定分区中为行分配唯一 ROW_NUMBER 的顺序。
参数:<partition_by_clause> :将 FROM 子句生成的结果集划入应用了 ROW_NUMBER 函数的分区。
<order_by_clause>:确定将 ROW_NUMBER 值分配给分区中的行的顺序。
返回类型:bigint 。示例:
/*以下示例将根据年初至今的销售额,返回 AdventureWorks 中销售人员的 ROW_NUMBER。*/USE AdventureWorks
GO
SELECT c.FirstName, c.LastName, ROW_NUMBER() OVER(ORDER BY SalesYTD DESC) AS 'Row Number', s.SalesYTD, a.PostalCode
FROM Sales.SalesPerson s JOIN Person.Contact c on s.SalesPersonID = c.ContactID
JOIN Person.Address a ON a.AddressID = c.ContactID
WHERE TerritoryID IS NOT NULL AND SalesYTD <> 0
/*
FirstName LastName Row Number SalesYTD PostalCode
--------- ---------- ---------- ------------ ----------------------------
Shelley Dyck 1 5200475.2313 98027
Gail Erickson 2 5015682.3752 98055
Maciej Dusza 3 4557045.0459 98027
Linda Ecoffey 4 3857163.6332 98027
Mark Erickson 5 3827950.238 98055
Terry Eminhizer 6 3587378.4257 98055
Michael Emanuel 7 3189356.2465 98055
Jauna Elson 8 3018725.4858 98055
Carol Elliott 9 2811012.7151 98027
Janeth Esteves 10 2241204.0424 98055
Martha Espinoza 11 1931620.1835 98055
Carla Eldridge 12 1764938.9859 98027
Twanna Evans 13 1758385.926 98055
(13 行受影响)
*/
/*以下示例将返回行号为 50 到 60(含)的行,并以 OrderDate 排序。*/
USE AdventureWorks;
GO
WITH OrderedOrders AS
(SELECT SalesOrderID, OrderDate,
ROW_NUMBER() OVER (order by OrderDate)as RowNumber
FROM Sales.SalesOrderHeader )
SELECT *
FROM OrderedOrders
WHERE RowNumber between 50 and 60;
/*
SalesOrderID OrderDate RowNumber
------------ ----------------------- --------------------
43708 2001-07-03 00:00:00.000 50
43709 2001-07-03 00:00:00.000 51
43710 2001-07-03 00:00:00.000 52
43711 2001-07-04 00:00:00.000 53
43712 2001-07-04 00:00:00.000 54
43713 2001-07-05 00:00:00.000 55
43714 2001-07-05 00:00:00.000 56
43715 2001-07-05 00:00:00.000 57
43716 2001-07-05 00:00:00.000 58
43717 2001-07-05 00:00:00.000 59
43718 2001-07-06 00:00:00.000 60
(11 行受影响)
*/