- 对数据分组的总结
举例：统计各个部门的平均工资，并且是大于1000的，并且按照平均工资从高到底排序
mysql> select avg(stsal) as myavgsal, stdepno from staff group by stdepno having myavgsal > 1000 order by myavgsal desc;
+-------------+---------+
| myavgsal | stdepno |
+-------------+---------+
| 7000.200000 | 10 |
| 5350.200000 | 30 |
| 3094.766667 | 20 |
+-------------+---------+
- 笛卡尔积的原理
多表查询练习： - 显示员工名，工资，及所在部门的名字
select staff.stname , staff.stsal, department.dname from staff, department where department.deptno = staff.stdepno;
- 显示部门号为10的部门名，员工名，工资，
select staff.stname , staff.stsal, department.dname from staff, department where department.deptno = staff.stdepno and department.deptno = 10;
- 显示每个员工的姓名，工资， 及其工资的级别
select staff.stname, staff.stsal, salalevel.slevel from staff, salalevel where staff.stsal >= slower and staff.stsal <= shig;
- 显示每个员工姓名，工资，以及部门名，以及工资级别
这样就设计到三张表的关联查询，三张表为了避免笛卡尔积， 就需要至少两个查询条件
select staff.stname, staff.stsal , department.dname, salalevel.slevel from staff,department,salalevel where staff.stdepno = department.deptno and staff.stsal >= salalevel.slower and staff.stsal <= salalevel.shig;
