group by使用注意事项：

执行下面查询语句：

select username, class from user group by username;

会报错：

Expression #2 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'test007.user.class' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by

正确解法：
select的查询子句，也就是上面的username、class，必须在group by子句里存在

where子句在group by子句前面

各种子句的执行顺序

from -> where -> group by -> select -> order by -> limit

where子句中不能有聚合函数（count、avg、sum）

having子句可加入聚合函数，它是放在group子句后

聚合查询通常跟group by一起使用

SELECT
    d.department_name,
    count(*)
FROM
    departments d
LEFT JOIN employees e ON d.department_id = e.department_id
GROUP BY
    d.department_name;