有这样一个AAA表如下所示:
name score color
jim 10 red
jim 20 blue
jim 20 green
jim 1 black
glin 2 red
glin 33 blue
glin 21 green
glin 19 black
bob 22 red
bob 39 blue
bob 11 green
bob 11 black
要转置成如下所示的BBB表
name red blue green black
jim 10 20 20 1
bob 22 39 11 11
glin 2 33 21 19
使用的sql如下:
select R.name ,R.s red,B.s blue,G.s green ,Bk.s black
from( select name,sum(score)s from AAA where color='red' group by name ) R,
( select name,sum(score)s from AAA where color='blue' group by name ) B,
( select name,sum(score)s from AAA where color='green' group by name ) G,
( select name,sum(score)s from AAA where color='black' group by name ) Bk
where R.name = B.name and B.name = G.name and G.name = Bk.name
相当于group by之后把每个人的某一种颜色的分数统计成一个数据集,再把这几个响应的数据集做表连接拼起来。
如果反过来,指导BBB表,要返回成AAA表,sql的写法是怎样呢?
select name, red score,'red' color from BBB
union
select name, blue score,'blue' color from BBB
union
select name, green score,'green' color from BBB
union
select name, black score,'black' color from BBB
就是把几个查询集并起来。