mysql學習筆記14 多表查詢初步

1. 對數據分組的總結

舉例：統計各個部門的平均工資，並且是大於1000的，並且按照平均工資從高到底排序

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 |
+-------------+---------+

2. 笛卡爾積的原理
   
   多表查詢練習：
3. 顯示員工名，工資，及所在部門的名字

select staff.stname , staff.stsal, department.dname from staff, department where department.deptno = staff.stdepno;

4. 顯示部門號爲10的部門名，員工名，工資，

select staff.stname , staff.stsal, department.dname from staff, department where department.deptno = staff.stdepno and department.deptno = 10;

5. 顯示每個員工的姓名，工資， 及其工資的級別

select staff.stname, staff.stsal, salalevel.slevel from staff, salalevel where staff.stsal >= slower and staff.stsal <= shig;

6. 顯示每個員工姓名，工資，以及部門名，以及工資級別
   這樣就設計到三張表的關聯查詢，三張表爲了避免笛卡爾積， 就需要至少兩個查詢條件

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;