语法顺序：
select distinct 字段1，字段2，字段3 from 库.表
where 条件
group by 分组条件
having 过滤 # 执行顺序的话，到这步会返回运行select语句，之后再运行order by
order by 排序字段
limit n； # 限制打印到屏幕上的条数
where 须在group by之前，不可用聚合函数
hanving需在group by之后，可用聚合函数
select 后面可以用聚合函数，如select count(id) from employee
执行顺序：
from->where->group by->having->distinct->order by->limit
# group by
设置 mysql> set global sql_mode="ONLY_FULL_GROUP_BY"; #select只能取分组的字段
select post from employee group by post;而不能select * from employee group by post;
# group_concat
把group by出来的记录的某个字段值连起来，如 select post,group_concat(name) from employee group by post;
# having
select * from employee group by post having count(id)<2 ;
# order by
select * from employee order by age desc,id asc;
# limit
select * from employee limit 0,5;
select * from employee limit 5,5; # 分页