学习Microsoft SQL Server 2008技术内幕：T-SQL语法基础--第4章

第4章 子查询

4.2.1 Exist 谓语：

use TSQLFundamentals2008
select *
from Sales.Customers as C
where c.country=N'Spain'

select *
from Sales.Customers as C
where c.country=N'Spain' and exists(select * from Sales.Orders as O where o.custid=C.custid)

select *
from Sales.Customers as C
where c.country=N'Spain' and not exists(select * from Sales.Orders as O where o.custid=C.custid)

4.3.2 连续聚和

select OBJECT_ID('Sales.OrderTotalsByYear')
if OBJECT_ID('Sales.OrderTotalsByYear') is not null  drop view Sales.OrderTotalsByYear

go
create view Sales.OrderTotalsByYear with schemabinding
as
  select
    YEAR(o.orderdate) as orderyear,
    SUM(od.qty) as qty
  from
  Sales.Orders as o
    join Sales.OrderDetails as od on o.orderid=od.orderid
  group by YEAR(o.orderdate)
go

查询：

select O1.orderyear, o1.qty,
    (select  SUM(o2.qty) from
      Sales.OrderTotalsByYear as o2
      where o2.orderyear <= o1.orderyear
    ) as runqty
from  Sales.OrderTotalsByYear as O1
order by o1.orderyear

4.3.3 行为不当的子查询

SELECT custid, companyname
FROM Sales.Customers AS C
WHERE custid NOT IN(SELECT O.custid
                    FROM Sales.Orders AS O);

返回：

custid   companyname
22        Customer DTDMN
57        Customer WVAXS

插入一条custid=NULL的记录：

INSERT INTO Sales.Orders
  (custid, empid, orderdate, requireddate, shippeddate, shipperid,
   freight, shipname, shipaddress, shipcity, shipregion,
   shippostalcode, shipcountry)
  VALUES(NULL, 1, '', '',
         '', 1, 123.00, N'abc', N'abc', N'abc',
         N'abc', N'abc', N'abc');

SELECT custid, companyname
FROM Sales.Customers AS C
WHERE custid NOT IN(SELECT O.custid
                    FROM Sales.Orders AS O);

返回0行

原因：

NOT(22=1 Or 22=2 Or 22=NULL)

NOT(False or False Or UnKnown)

NOT UnKnow

Unknow

查询条件过滤UnKnown

而：

SELECT custid, companyname
FROM Sales.Customers AS C
WHERE  not exists (SELECT *
                    FROM Sales.Orders AS O where C.custid=O.custid);

或

SELECT custid, companyname
FROM Sales.Customers AS C
WHERE custid NOT IN(SELECT O.custid
                    FROM Sales.Orders AS O where O.custid is NOT null);

返回：

custid   companyname
22        Customer DTDMN
57        Customer WVAXS