MySQL(分组、连表操作、备份数据库)

day58

分组

参考：https://www.cnblogs.com/xp796/p/5262187.html

select dept, max(salary) from department group by dept;

从各个部门中挑出最高的薪水。

select dept, sum(salary) from department group by dept;

计算各个部门的总薪水。

select count(id), edlevel from department group by edlevel;

不同等级员工的数量。

select max(salary),dept from department group by dept having avg(salary)<3000;（条件）

从平均工资大于3000的部门中，挑出部门中最高的薪水。

连表操作

连表前两表有外键约束。

select * from course,teacher;

左右表都显示，但是两表之间联系并未体现。

select * from course,teacher where course.teacher_id = teacher.tid;
连表的第一种用法，teacher_id与tid对应合并。

# select * from teacher right join course on teacher.tid = course.teacher_id
# course右边全部显示(课的所有信息显示)

select * from teacher left join course on teacher.tid = course.teacher_id
select * from course left join teacher on teacher.tid = course.teacher_id
# teacher左边全部显示 (各个老师全部显示，尽管有些课没有该老师上课，没有课上的老师显示null)

select * from teacher innder join course on teacher.tid = course.teacher_id;
将出现null时一行隐藏