SQL转置 （CASE）

SELECT (CASE WHEN startdate >= '2003-12-31' 
             THEN startdate ELSE '2003-12-31' END) as thedate,
	market, COUNT(*) as numstarts, 0 as numstops
FROM subscribers s
WHERE startdate IS NOT NULL
GROUP BY (CASE WHEN startdate >= '2003-12-31' 
	       THEN startdate ELSE '2003-12-31' END), market
ORDER BY thedate

计算三个市场中每天开始的订阅客户数量。利用子查询分组，将早于2003-12-31的订单归为一组。以thedate和market字段分组。得到

thedate		market		numstarts    numstops
2003-12-31	Gotham		1049470		0
2003-12-31	Metropolis	588557		0
2003-12-31	Smallville	368107		0
2004-01-01	Gotham		218		0
2004-01-01	Metropolis	129		0
2004-01-01	Smallville	2		0

用UNION ALL 联合结束的人数。最后在外层查询中用CASE将每一个市场拆分出来从而完成转置。