mysql ---排序、分组、并集操作、distinct

1、order  by 排序

select * from employee order by salary ASC limit 10; //查询出按salary排序最低的的10,名员工

2.distinct 获取不重复的唯一值

select distinct first_name from employee;

3.group by 分组统计

select first_name,count(*) cnt from employee group by first_name order by cnt DESC; // 按照first_name分组，并根据first_name出现次数按降序排列

可以在group by 语句后添加 having 字句，对聚集结果进行筛选

select first_name,count(*) cnt from employee group by first_name having cnt > 10 order by cnt desc;

4.union 和 union all

select * from a union select * from b;

select * from a union all select * from b;

上面两个语句都是执行并集操作，但union all 比union更快，union 实际上是union distinct，在进行表连接的时候会筛选掉重复记录，所以在表连接后会对产生的结果集进行排序运算，删除重复的记录再返回结果。