SQL SERVER中CTE语句结构及CTE递归查询 CTE语句结构 公用表表达式 (CTE) 可以认为是在单个 SELECT、INSERT、UPDATE、DELETE 或 CREATE VIEW 语句的执行范围内定义的临时结果集。CTE 与派生表类似,具体表现在不存储为对象,并且只在查询期间有效。与派生表的
SQL SERVER中CTE语句结构及CTE递归查询
CTE语句结构
公用表表达式 (CTE) 可以认为是在单个 SELECT、INSERT、UPDATE、DELETE 或 CREATE VIEW 语句的执行范围内定义的临时结果集。CTE 与派生表类似,具体表现在不存储为对象,并且只在查询期间有效。与派生表的不同之处在于,CTE 可自引用,还可在同一查询中引用多次。
使用上面的test4表为例:
WITH TEST_CTE
AS
(
select * from test4
)
此句创建了名为TEST_CTE的select * from test4的结果集。由于它不存储为对象,并且只在查询期间有效,所CTE和查询语句需要在一起执行:
WITH TEST_CTE
AS
(
select * from test4
)
select * from TEST_CTE
结果集跟select * from test4结果集机同。
下面是指定列的CTE使用:
WITH TEST_CTE(id)
AS
(
select id from test4
)
定义中的列需要与语句里面的列对应,,见红色字体。
CTE可用于创建递归查询。
创建测试表并插入数据:
create table test5
(
id int,
name varchar(50),
parentid int
)
insert into test5(id,name,parentid)
select 1,'父类1',0
union all
select 2,'父类2',0
union all
select 3,'父类3',0
union all
select 11,'子类11',1
union all
select 12,'子类12',1
union all
select 111,'子子类111',11
union all
select 22,'子类22',2
union all
select 222,'子子类222',22
结果:
id name parentid
1 父类1 0
2 父类2 0
3 父类3 0
11 子类11 1
12 子类12 1
111 子子类111 11
22 子类22 2
222 子子类222 22
使用CTE创建递归查询,获取父类1及所有其子类及子类的子类...:
with Test_Recursion(id,name,parentid,[level])
AS
(
select id,name,parentid,0 from test5 where id =1--没有引用CTE自身必须放在第一个递归行之上
union all--没有引用CTE自身的语句和第一个递归行之间必须用UNION ALL
select a.id,a.name,a.parentid,b.[level]+1 from test5 as a join Test_Recursion as b on a.parentid=b.id--递归行
)
select * from Test_Recursion
结果:
id name parentid level
1 父类1 0 0
11 子类11 1 1
12 子类12 1 1
111 子子类111 11 2

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