SQL Server新特性总结复习（一）(二)
int = 0
虽然有时很快捷，但DBA不建议这样使用，Declare @id = select top 1 id from 表名，建议声明和查表赋值分开
9. 公用表达式 CTE
特点：可嵌套使用，代替联接表中的子查询，结构层次更加清晰，也可用来递归查询，另外通过巧妙的常量列控制递归层次
示例代码如下：
使用CTE
--公用表达式CTE Common table expression
--用CTE实现递归算法
CREATE TABLE EMPLOYEETREE(
EMPLOYEE INT PRIMARY KEY,
employeename nvarchar(50),
reportsto int
)
insert into EMPLOYEETREE values(1,'Richard',null)
insert into EMPLOYEETREE values(2,'Stephen',1)
insert into EMPLOYEETREE values(3,'Clemens',2)
insert into EMPLOYEETREE values(4,'Malek',2)
insert into EMPLOYEETREE values(5,'Goksin',4)
insert into EMPLOYEETREE values(6,'Kimberly',1)
insert into EMPLOYEETREE values(7,'Ramesh',5)
----------------------
--确定哪些员工向Stephen报告的递归查询
with employeeTemp as
(
select EMPLOYEE, employeename, reportsto from EMPLOYEETREE where EMPLOYEE = 2
union all
select a.EMPLOYEE, a.employeename, a.reportsto from EMPLOYEETREE as a
inner join employeeTemp as b on a.reportsto = b.EMPLOYEE
)
select * from employeeTemp where EMPLOYEE <> 2 --option(maxrecursion 2)
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--不报错设置级联关联递归
with employeeTemp as
(
select EMPLOYEE, employeename, reportsto,0 as sublevel from EMPLOYEETREE where EMPLOYEE = 2
union all
select a.EMPLOYEE, a.employeename, a.reportsto,sublevel+1 from EMPLOYEETREE as a
inner join employeeTemp as b on a.reportsto = b.EMPLOYEE
)
select * from employeeTemp where EMPLOYEE <> 2 and sublevel <=2 --option(maxrecursion 2)
10. pivot 与 unpivot
前者用在行转列，注意：必须用聚合函数与PIVOT一起使用，计算聚会时将不考虑出现在值列中的任何空值；一般情况下，可以用列上的子查询来替换pivot语句，但是这样做效率不高
后者用在列转行，注意：如果某些列中有null值，将会被过滤掉，不产生新行；语法上For前指定的新列，对应原表指定列名中的值，For后指定的新列对应原表指定列名中的标题的值
两者都有的共性：语法上最后必须要有别名；IN里面指定的列类型必须是一致的。
示例代码如下：
pivot与unpivot
--关于PIVOT的操作
CREATE TABLE #test
(
NAME VARCHAR(max),
SCORE INT
) www.2cto.com
INSERT INTO #test VALUES ('张三','97')
INSERT INTO #test VALUES ('李四','28')
INSERT INTO #test VALUES ('王五','33')
INSERT INTO #test VALUES ('神人','78')
--NAME SCORE
--张三 97
--李四 28
--王五 33
--神人 78
--行转列
SELECT --'成绩单' AS SCORENAME ,
[张三], [李四], [王五]
FROM #test
PIVOT (AVG(SCORE) FOR NAME IN ([张三], [李四], [王五])) b
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-----------------------------------------
CREATE TABLE VendorEmployee(
VendorId INT,
Emp1Order INT,
Emp2Order INT,
Emp3Order INT,
Emp4Order INT,
Emp5Order INT,
)
GO
INSERT INTO VendorEmployee VALUES(1,4,3,5,4,4)
INSERT INTO VendorEmployee VALUES(2,4,1,5,5,5)
INSERT INTO VendorEmployee VALUES(3,4,3,5,4,4)
INSERT INTO VendorEmployee VALUES(4,4,2,5,4,4)
INSERT INTO VendorEmployee VALUES(5,5,1,5,5,5)
SELECT * FROM VendorEmployee
----------------
--列转行
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SELECT * FROM (
SELECT VendorId,[Emp1Order],[Emp2Order],[Emp3Order],[Emp4Order],[Emp5Order] FROM VendorEmployee) AS unpiv
UNPIVOT (orders FOR elyid IN ([Emp1Order],[Emp2Order],[Emp3Order],[Emp4Order],[Emp5Order])) AS child
ORDER BY elyid
SELECT * FROM VendorEmployee
UNPIVOT (orders FOR elyid IN ([Emp1Order],[Emp2Order],[Emp3Order],[Emp4Order],[Emp5Order])) AS child
ORDER BY elyid
SELECT * FROM VendorEmployee UNPIVOT ( ORDERS FOR [操作员名字] IN ([Emp1O