sql语句之查询操作

 

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