mysql语句的语法模板：
select distinct <select_list>
from
<left_table><join_type> join <right_table> on <join_condition>
where <where_condition>
group by <group_by_rowname>
having <having_condition>
order by <order_by_condition>
limit <limit_number>
mysql的执行顺序如图：
mytest表 如图：
想先根据department字段分组，在按照price字段排序：SELECT * FROM mytest ORDER BY department ,price
想查询每个department字段的最大price值时：SELECT id ,department,MAX(price) FROM mytest GROUP BY department
多表关联查询时,表结构如图：
user表
pay 表
spay表：
其中pay表 是用户缴费表，spay表 是应该缴费表
想要查询用户欠费的记录时查询如下：
SELECT spay.userId, (T.mypay-spay.shouldPay) AS own FROM
(SELECT userId,SUM(mypay) AS mypay FROM pay GROUP BY userId) AS T LEFT JOIN spay ON spay.`userId`=T.userId
注意：有时候需要用到关键字union关键字，union是合并+去重的操作