07 分组

select age,max(id),min(id),count(id),sum(id),avg(id) from info group by age;

select age,count(1) from info group by age;

select depart_id,count(id) from info group by depart_id;

select depart_id,count(id) from info group by depart_id having count(id) > 2;

select count(id) from info;
select max(id) from info;

select age,max(id),min(id),sum(id),count(id) from info group by age;

select age,name from info group by age;  -- 不建议
select * from info where id in (select max(id) from info group by age);

select age,count(id) from info group by age having count(id) > 2;
select age,count(id) from info where id > 4 group by age having count(id) > 2;  -- 聚合条件放在having后面

到目前为止SQL执行顺序：
    where 
    group by
    having 
    select
    order by
    limit

select age,count(id) from info where id > 2 group by age having count(id) > 1 order by age desc limit 1;
- 要查询的表info
- 条件 id>2
- 根据age分组
- 对分组后的数据再根据聚合条件过滤 count(id)>1
- 根据age从大到小排序
- 获取第1条