【MySQL】十八、where,from,select後面巢狀子查詢
阿新 • • 發佈:2021-02-19
技術標籤:MySQL
文章目錄
1. 什麼是子查詢?子查詢都可以出現在哪裡?
select 語句當中巢狀select語句,被巢狀的select語句時子查詢。
子查詢可以出現在哪裡?
select
...(select)...
from
...(select)...
where
...(select)...
2. where子句中巢狀子查詢
案例:找出高於平均薪資的員工資訊。
select * from emp where sal > avg(sal); //錯誤
以上是一種錯誤的寫法,牢記:where後面不能直接使用分組函式
正確寫法:
第一步:找出平均薪資;
select avg(sal) from emp;
+-------------+
| avg(sal) |
+-------------+
| 2073.214286 |
+-------------+
第二步:where過濾
select * from emp where sal > 2073.214286 ;
+-------+-------+-----------+------+------------+---------+------+--------+
| EMPNO | ENAME | JOB | MGR | HIREDATE | SAL | COMM | DEPTNO |
+-------+-------+-----------+------+------------+---------+------+--------+
| 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 |
| 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850.00 | NULL | 30 |
| 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450.00 | NULL | 10 |
| 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL | 20 |
| 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 |
| 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 |
+-------+-------+-----------+------+------------+---------+------+--------+
6 rows in set (0.00 sec)
第一步和二合併步(在where子句中使用了子查詢):
select * from emp where sal > (select avg(sal) from emp);
查詢結果同上。
3. from子句中巢狀子查詢
案例1: 找出每個部門平均薪水的薪資等級
第一步:計算每個部門的平均工資(按照部門編號分組):
select deptno, avg(sal) as avgsal from emp group by deptno;
+--------+-------------+
| deptno | avgsal |
+--------+-------------+
| 10 | 2916.666667 |
| 20 | 2175.000000 |
| 30 | 1566.666667 |
+--------+-------------+
3 rows in set (0.00 sec)
第二步:對每個部門的平均工資進行分級:
我們可以將上面查詢出的結果當作一張臨時新表t,與表salgrade進行連線查詢,條件為
t.avgsal between s.losal and s.hisal
select
t.deptno, t.avgsal, s.grade
from
(select deptno, avg(sal) as avgsal from emp group by deptno) t
join
salgrade s
on
t.avgsal between s.losal and s.hisal;
+--------+-------------+-------+
| deptno | avgsal | grade |
+--------+-------------+-------+
| 10 | 2916.666667 | 4 |
| 20 | 2175.000000 | 4 |
| 30 | 1566.666667 | 3 |
+--------+-------------+-------+
3 rows in set (0.00 sec)
案例2: 找出每個部門平均的薪資等級。
第一步:求出每個部門每個員工的薪水等級
select
e.deptno, e.ename, e.sal, s.grade
from
emp e
join
salgrade s
on
e.sal between losal and hisal;
+--------+--------+---------+-------+
| deptno | ename | sal | grade |
+--------+--------+---------+-------+
| 20 | SMITH | 800.00 | 1 |
| 30 | ALLEN | 1600.00 | 3 |
| 30 | WARD | 1250.00 | 2 |
| 20 | JONES | 2975.00 | 4 |
| 30 | MARTIN | 1250.00 | 2 |
| 30 | BLAKE | 2850.00 | 4 |
| 10 | CLARK | 2450.00 | 4 |
| 20 | SCOTT | 3000.00 | 4 |
| 10 | KING | 5000.00 | 5 |
| 30 | TURNER | 1500.00 | 3 |
| 20 | ADAMS | 1100.00 | 1 |
| 30 | JAMES | 950.00 | 1 |
| 20 | FORD | 3000.00 | 4 |
| 10 | MILLER | 1300.00 | 2 |
+--------+--------+---------+-------+
14 rows in set (0.00 sec)
第二步:對上述結果看做一張新表t,對每個部門薪資等級的平均值(對部門進行分組求均值);
select
t.deptno, avg(t.grade)
from
(select e.deptno, e.ename, s.grade from emp e join salgrade s on e.sal between losal and hisal) t
group by
t.deptno;
+--------+--------------+
| deptno | avg(t.grade) |
+--------+--------------+
| 10 | 3.6667 |
| 20 | 2.8000 |
| 30 | 2.5000 |
+--------+--------------+
3 rows in set (0.00 sec)
但是,這裡實際上沒必要把新表看做一張臨時表,這裡其實可以直接寫:
select
e.deptno, avg(s.grade)
from
emp e
join
salgrade s
on
e.sal between losal and hisal
group by
deptno;
查詢結果與上述一致:
+--------+--------------+
| deptno | avg(s.grade) |
+--------+--------------+
| 10 | 3.6667 |
| 20 | 2.8000 |
| 30 | 2.5000 |
+--------+--------------+