《SQL 进阶教程》 case

SQL 权威指南
SQL 解惑
在进行非定制化统计时，需要将已有编号方式转换为另外一种便于分析的方式进行统计需求

select
case
when name='哈尔滨' then '黑龙江'
when name='大庆' then '黑龙江'
when name='齐齐哈尔' then '黑龙江'
when name='长春' then '吉林'
when name='吉林' then '吉林'
when name='沈阳' then '辽宁'
when name='大连' then '辽宁'
when name='葫芦岛' then '辽宁'
else '其他'
end as province,sum(population)
from city_population
GROUP BY
case
when name='哈尔滨' then '黑龙江'
when name='大庆' then '黑龙江'
when name='齐齐哈尔' then '黑龙江'
when name='长春' then '吉林'
when name='吉林' then '吉林'
when name='沈阳' then '辽宁'
when name='大连' then '辽宁'
when name='葫芦岛' then '辽宁'
else '其他' end

select
case
when name='哈尔滨' then '黑龙江'
when name='大庆' then '黑龙江'
when name='齐齐哈尔' then '黑龙江'
when name='长春' then '吉林'
when name='吉林' then '吉林'
when name='沈阳' then '辽宁'
when name='大连' then '辽宁'
when name='葫芦岛' then '辽宁'
else '其他'
end as province,sum(population)
from city_population
group by province

严格来说，这种写法违反标准sql规范，因为Group by 比select 语句先知，
所以group by 子句中引用在select 子句里定义的别称是不被允许的，
事实上，在oracle、DB2、SQLServer 等数据库采用这种写法时就会出错

将人口按照适当的级别进行分类统计
select
case
when population < 100 then '1'
when population >=100 and population <200 then '2'
when population >=200 and population < 400 then '3'
else '4' end as pop_class,count(*) as cnt
from city_population
GROUP BY pop_class

进行不同条件的统计是case表达式的著名用法之一

select
name,
sum(case when sex = 1 then population else 0 end) as cnt_m,
sum(case when sex = 2 then population else 0 end) as cnt_f
from city_population_copy
group by name

实现了行转列