sql 学习

一、如何从select的查询结果中再次运算？
第一步：
粗查询。首先需要将第一层查询弄对，关系弄清楚。
select sum(quantity) total ,price_water,price_pollute from pay_water where time_pay >='2010-02-01 00:00:00.000' and time_pay <'2010-02-28 23:59:59.000'
group by price_water,price_pollute
第二步：
二次查询。在一次查询的基础上进行运算。
select total*price_water a ,total*price_pollute b  from (
select sum(quantity) total ,price_water,price_pollute from pay_water where time_pay >='2010-02-01 00:00:00.000' and time_pay <'2010-02-28 23:59:59.000'
group by price_water,price_pollute
) as test
as test是必须有的，没有就会报错。
第三步：
压缩：
select sum(total*price_water) a ,sum(total*price_pollute) b  from (
select sum(quantity) total ,price_water,price_pollute from pay_water where time_pay >='2010-02-01 00:00:00.000' and time_pay <'2010-02-28 23:59:59.000'
group by price_water,price_pollute
) as test
sum()函数可以将上下压缩，求出该列的总和。
二、group by 语句。
select中的字段只能是group by中的字段或者聚合函数。