sql之表的表达式

1、派生表

实质：就是特殊的子查询（将查询结果放在from后面）

含有一张Order表：

看下面的sql语句：

1 select

2 orderid,orderdate,custid

3 from

4 (

5    select

6    orderid,orderdate,custid,ROW_NUMBER() over(order by orderid) as rownum--列明必须起别名

7    from  [Sales.Orders]

8 ) as t--表名必须起别名

需要注意的特殊之处：查询出来的表要起别名，子查询里面的字段名也要起别名

2、CTE（公共表的表达式）

 1）语法规范：

1 with use_country--定义的表名

2 as--上面的是 语法：

3 (

4     select country,companyname,custid

5     from [Sales.Customers]

6     where country='按时打算'

7 )

8 select * from use_country;--将查询语句 写在 子查询的后面

将查询语句放在最后。

下面看几个例子，具体解析一下：

业务逻辑：还是用上面的那张Order表，要求：查询出 每年的订单数量 大于10的 用户

1》普通的写法：

1 select

2 custid,year(orderdate),COUNT(1) as '订单数量'

3 from [Sales.Orders]

4 group by YEAR(orderdate), custid

5 having COUNT(orderid)>10--having:对 group by  之后的 组函数 进行 筛选使用

 

 2》使用派生表：

 1 select

 2 custid,orderyear,orderNum

 3 from

 4 (

 5     select

 6     custid,orderyear,COUNT(*) as orderNum

 7     from

 8     (

 9         select

10         custid, year(orderdate) as orderyear

11         from [Sales.Orders]

12     ) as t1

13     group by orderyear,custid

14 ) as t2

15 where orderNum>10

上面的 查询的层次只有两层，如果层次非常多的话，就会出现，看起来非常费劲了，所以使用CTE的方式更简洁

 1 with OrderYear

 2 as

 3 (

 4     select

 5     custid,YEAR(orderdate) as orderyear

 6     from [Sales.Orders]

 7 ),

 8 OrderGroupYear

 9 as

10 (

11   select

12   custid,orderyear,COUNT(orderyear) as ordernum

13   from OrderYear

14   group by orderyear,custid

15 ),

16 OrderNumThanTen

17 as

18 (

19   select

20   custid,orderyear,ordernum

21   from OrderGroupYear

22   where ordernum>10

23 )

24 select * from OrderNumThanTen;

 

2）CTE还可以多张表引用：（相当于c#里面的将重复的代码封装成一个方法）下面举例：

业务逻辑：查询出 每年 客户的数量，以及 前后两年之间客户数量的差量

--首先查询出每年叫客户的数量：

1 select

2 year(orderdate),COUNT(distinct custid)--将 重复的 客户 要去掉

3 from [Sales.Orders]

4 group by YEAR(orderdate)

--然后使用派生表的方式实现：

 1 select

 2 currtYearOrder.orderYear,currtYearOrder.custCount,prevYearOrder.orderYear,prevYearOrder.custCount,currtYearOrder.custCount-prevYearOrder.custCount

 3 from

 4 (

 5     select

 6     year(orderdate) as orderYear,COUNT(distinct custid) as custCount--将 重复的 客户 要去掉

 7     from [Sales.Orders]

 8     group by YEAR(orderdate)

 9 ) as currtYearOrder

10 left outer join 

11 (

12     select

13     year(orderdate) as orderYear ,COUNT(distinct custid) as custCount--将 重复的 客户 要去掉

14     from [Sales.Orders]

15     group by YEAR(orderdate)

16 ) as prevYearOrder

17 on currtYearOrder.orderYear=prevYearOrder.orderYear+1

 

--下面使用CTE的方式实现，就不用重复：

 1 with OrderYearCust

 2 as

 3 (

 4     select

 5     year(orderdate) as orderYear,COUNT(distinct custid) as custCount--将 重复的 客户 要去掉

 6     from [Sales.Orders]

 7     group by YEAR(orderdate)

 8 )

 9 select 

10 curtYearOrder.orderYear,curtYearOrder.custCount,prevYearOrder.orderYear,prevYearOrder.custCount,curtYearOrder.custCount-prevYearOrder.custCount

11 from OrderYearCust as curtYearOrder left outer join OrderYearCust as prevYearOrder on curtYearOrder.orderYear=prevYearOrder.orderYear+1

 

3）CTE递归查询：（针对 树形节点 进行查询）

表的结果如下：

业务逻辑：需要查询出 mgrid 为 2 下面的 所有的子节点

 1 with diguiEmployee

 2 as

 3 (

 4    --起点:最上层的 查询(只执行一次)

 5     select 

 6     enpid,lastname,firstname,mgrid

 7     from [HR.Employees]

 8     where mgrid=6

 9    

10     union all --连接 起点:上层查询 和递归查询

11     

12     --递归查询

13     select 

14     e.enpid,e.lastname,e.firstname,e.mgrid

15     from [HR.Employees] as e inner  join diguiEmployee as d

16     on e.mgrid=d.enpid

17 )

18 select     enpid,lastname,firstname,mgrid from [HR.Employees] where enpid=6

19 union all --将查询的 结果 连接 起来

20 select * from diguiEmployee;