Pivot

Pivot可以使sql表中多行旋转为列，UnPivot则使多列旋转为多行
有两个表Customers和Orders，内容如下：

select * from Customers
--结果如下
CustomerId, City
FISSA		Madrid
FRNDO		Madrid
KRLOS		Madrid
MRPHS		ZION

select * from Orders
--结果如下
orderid, customerid
1			FRNDO
2			FRNDO
3			KRLOS
4			KRLOS
5			KRLOS
6			MRPHS
7			NULL

现想找出每个城市中，订单数为0和非0的个数，可使用如下sql，使用Pivot：

select city,zeroorder,haveorder from
(select c.customerid,c.city,
case when count(orderid)=0 then 'ZeroOrder'
	 when count(orderid)>0 then 'HaveOrder'
end as category
from customers c left outer join orders o on c.customerid = o.customerid
group by c.customerid,c.city) d
pivot(
count(customerid) 
for category in ([ZeroOrder],[HaveOrder])
) as p
order by city desc

以下sql，没有使用Pivot，和上边使用Pivot的情况等价：

select city,
count(case when category = 'ZeroOrder' then customerid end) as zeroorder,
count(case when category = 'HaveOrder' then customerid end) as HaveOrder
from
(
select c.customerid,c.city,
case when count(orderid)=0 then 'ZeroOrder'
	 when count(orderid)>0 then 'HaveOrder'
end as category
from customers c left outer join orders o on c.customerid = o.customerid
group by c.customerid,c.city
)
d
group by city
order by city desc