EMP+DEPT+SALGRADE 表的基本操作2
阿新 • • 發佈:2020-07-13
distinct 去重
關於查詢結果集的去重
distinct
mysql> SELECT distinct job from EMP;
+-----------+
| job |
+-----------+
| CLERK |
| SALESMAN |
| MANAGER |
| ANALYST |
| PRESIDENT |
+-----------+
5 rows in set (0.00 sec)
統計崗位的數量
mysql> select count(distinct job) from EMP; +---------------------+ | count(distinct job) | +---------------------+ | 5 | +---------------------+ 1 row in set (0.00 sec)
連線查詢
什麼是連線查詢
在實際開發中,大部分的情況都不是從一張表中查詢資料,一般都是多張表聯合查詢去出最終的結果
找出每一個員工的部門名稱,要求顯示員工名和部門名
會用到 別名
- 別名有什麼好處?
- 第一:執行效率高
- 第二:可讀性好
mysql> select e.ename,e.deptno from EMP e ,DEPT d where e.deptno = d.deptno; +--------+--------+ | ename | deptno | +--------+--------+ | CLARK | 10 | | KING | 10 | | MILLER | 10 | | SMITH | 20 | | JONES | 20 | | SCOTT | 20 | | ADAMS | 20 | | FORD | 20 | | ALLEN | 30 | | WARD | 30 | | MARTIN | 30 | | BLAKE | 30 | | TURNER | 30 | | JAMES | 30 | +--------+--------+ 14 rows in set (0.00 sec)
內連線之等值連線
最大特點是:條件是等量關係
查詢每個員工的部門名稱,要求顯示員工名和部門名
找出每一個員工的部門名稱,要求顯示員工名和部門名
表A join 表B ... on(連線條件)
好處:連線條件後面可以加where判斷
mysql> select e.ename,d.deptno from EMP e join DEPT d on e.deptno = d.deptno; +--------+--------+ | ename | deptno | +--------+--------+ | CLARK | 10 | | KING | 10 | | MILLER | 10 | | SMITH | 20 | | JONES | 20 | | SCOTT | 20 | | ADAMS | 20 | | FORD | 20 | | ALLEN | 30 | | WARD | 30 | | MARTIN | 30 | | BLAKE | 30 | | TURNER | 30 | | JAMES | 30 | +--------+--------+ 14 rows in set (0.00 sec) ---------------------------------加上where--------------------------- mysql> select e.ename,d.deptno from EMP e join DEPT d on e.deptno = d.deptno where d.deptno=10; +--------+--------+ | ename | deptno | +--------+--------+ | CLARK | 10 | | KING | 10 | | MILLER | 10 | +--------+--------+ 3 rows in set (0.00 sec)
找出每個員工的工資等級,要求顯示員工名,工資,工資等級
mysql> select e.ename,e.sal,s.grade from EMP e join SALGRADE s on e.sal between s.losal and s.hisal;
+--------+------+-------+
| ename | sal | grade |
+--------+------+-------+
| SMITH | 800 | 1 |
| ALLEN | 1600 | 3 |
| WARD | 1250 | 2 |
| JONES | 2975 | 4 |
| MARTIN | 1250 | 2 |
| BLAKE | 2850 | 4 |
| CLARK | 2450 | 4 |
| SCOTT | 3000 | 4 |
| KING | 5000 | 5 |
| TURNER | 1500 | 3 |
| ADAMS | 1100 | 1 |
| JAMES | 950 | 1 |
| FORD | 3000 | 4 |
| MILLER | 1300 | 2 |
+--------+------+-------+
14 rows in set (0.00 sec)
自連線
特點:一張表顯示兩張表,自己連線自己
找出每個員工的上級領導,要求顯示員工名和對應的領導名
mysql> select a.ename "員工名",b.ename "領導名" from EMP a inner join EMP b on a.empno = b.MGR;
+-----------+-----------+
| 員工名 | 領導名 |
+-----------+-----------+
| FORD | SMITH |
| BLAKE | ALLEN |
| BLAKE | WARD |
| KING | JONES |
| BLAKE | MARTIN |
| KING | BLAKE |
| KING | CLARK |
| JONES | SCOTT |
| BLAKE | TURNER |
| SCOTT | ADAMS |
| BLAKE | JAMES |
| JONES | FORD |
| CLARK | MILLER |
+-----------+-----------+
13 rows in set (0.00 sec)
外連線(左連線/右連線)
什麼是外連線,和內連線有什麼區別?
內連線:
假設A和B表進行連線,使用內連線的話,凡是A表和B表能夠匹配上的記錄查詢出來,這就是內連線
AB兩張表沒有主副之分,兩張表示平等的
外連線:
假設A和B表進行連線,使用外連線的話,AB兩張表中有一張是主表,一張是副表,主要查詢主表中的資料
,順便查詢副表中的資料沒有喝主表的資料匹配上,副表自動模擬出null與之匹配
找出每個員工的上級領導,要求顯示員工名和對應的領導名
mysql> select a.ename "員工編號",b.ename "大佬" from EMP a left join EMP b on a.empno= b.mgr;
+--------------+--------+
| 員工編號 | 大佬 |
+--------------+--------+
| FORD | SMITH |
| BLAKE | ALLEN |
| BLAKE | WARD |
| KING | JONES |
| BLAKE | MARTIN |
| KING | BLAKE |
| KING | CLARK |
| JONES | SCOTT |
| BLAKE | TURNER |
| SCOTT | ADAMS |
| BLAKE | JAMES |
| JONES | FORD |
| CLARK | MILLER |
| SMITH | NULL |
| ALLEN | NULL |
| WARD | NULL |
| MARTIN | NULL |
| TURNER | NULL |
| ADAMS | NULL |
| JAMES | NULL |
| MILLER | NULL |
+--------------+--------+
21 rows in set (0.00 sec)
mysql> select a.ename "員工編號",b.ename "大佬" from EMP a right join EMP b on a.empno= b.mgr;
+--------------+--------+
| 員工編號 | 大佬 |
+--------------+--------+
| FORD | SMITH |
| BLAKE | ALLEN |
| BLAKE | WARD |
| KING | JONES |
| BLAKE | MARTIN |
| KING | BLAKE |
| KING | CLARK |
| JONES | SCOTT |
| NULL | KING |
| BLAKE | TURNER |
| SCOTT | ADAMS |
| BLAKE | JAMES |
| JONES | FORD |
| CLARK | MILLER |
+--------------+--------+
14 rows in set (0.00 sec)
找出哪個部門沒有員工
mysql> select d.* from EMP e right join DEPT d on e.deptno=d.deptno where e.ename is null;
+--------+------------+--------+
| DEPTNO | DNAME | LOC |
+--------+------------+--------+
| 40 | OPERATIONS | BOSTON |
+--------+------------+--------+
1 row in set (0.00 sec)
找出每一個員工的部門名稱以及工資等級
涉及以下三張表
mysql> select * from EMP;
+-------+--------+-----------+------+------------+------+------+--------+
| EMPNO | ENAME | JOB | MGR | HIREDATE | SAL | COMM | DEPTNO |
+-------+--------+-----------+------+------------+------+------+--------+
| 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800 | NULL | 20 |
| 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600 | 300 | 30 |
| 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250 | 500 | 30 |
| 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975 | NULL | 20 |
| 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250 | 1400 | 30 |
| 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850 | NULL | 30 |
| 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450 | NULL | 10 |
| 7788 | SCOTT | ANALYST | 7566 | 1987-07-13 | 3000 | NULL | 20 |
| 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000 | NULL | 10 |
| 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500 | 0 | 30 |
| 7876 | ADAMS | CLERK | 7788 | 1987-07-13 | 1100 | NULL | 20 |
| 7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950 | NULL | 30 |
| 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000 | NULL | 20 |
| 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300 | NULL | 10 |
+-------+--------+-----------+------+------------+------+------+--------+
14 rows in set (0.00 sec)
mysql> select * from DEPT;
+--------+------------+----------+
| DEPTNO | DNAME | LOC |
+--------+------------+----------+
| 10 | ACCOUNTING | NEW YORK |
| 20 | RESEARCH | DALLAS |
| 30 | SALES | CHICAGO |
| 40 | OPERATIONS | BOSTON |
+--------+------------+----------+
4 rows in set (0.00 sec)
mysql> select * from SALGRADE;
+-------+-------+-------+
| GRADE | LOSAL | HISAL |
+-------+-------+-------+
| 1 | 700 | 1200 |
| 2 | 1201 | 1400 |
| 3 | 1401 | 2000 |
| 4 | 2001 | 3000 |
| 5 | 3001 | 9999 |
+-------+-------+-------+
5 rows in set (0.00 sec)
mysql> select e.ename,d.dname,s.grade from EMP e join DEPT d on e.deptno=d.deptno join SALGRADE s on e.sal between losal andd hisal;
+--------+------------+-------+
| ename | dname | grade |
+--------+------------+-------+
| SMITH | RESEARCH | 1 |
| ALLEN | SALES | 3 |
| WARD | SALES | 2 |
| JONES | RESEARCH | 4 |
| MARTIN | SALES | 2 |
| BLAKE | SALES | 4 |
| CLARK | ACCOUNTING | 4 |
| SCOTT | RESEARCH | 4 |
| KING | ACCOUNTING | 5 |
| TURNER | SALES | 3 |
| ADAMS | RESEARCH | 1 |
| JAMES | SALES | 1 |
| FORD | RESEARCH | 4 |
| MILLER | ACCOUNTING | 2 |
+--------+------------+-------+
14 rows in set (0.00 sec)
找出每一個員工的部門名稱,工資等級,以及上司領導
mysql> select e.ename "員工",d.dname,s.grade,e1.ename "領導" from EMP e join DEPT d on e.deptno=d.deptno join SALGRADE s on e.sal between losal and hisal left join EMP e1 on e1.empno = e.mgr;
+--------+------------+-------+--------+
| 員工 | dname | grade | 領導 |
+--------+------------+-------+--------+
| SMITH | RESEARCH | 1 | FORD |
| ALLEN | SALES | 3 | BLAKE |
| WARD | SALES | 2 | BLAKE |
| JONES | RESEARCH | 4 | KING |
| MARTIN | SALES | 2 | BLAKE |
| BLAKE | SALES | 4 | KING |
| CLARK | ACCOUNTING | 4 | KING |
| SCOTT | RESEARCH | 4 | JONES |
| KING | ACCOUNTING | 5 | NULL |
| TURNER | SALES | 3 | BLAKE |
| ADAMS | RESEARCH | 1 | SCOTT |
| JAMES | SALES | 1 | BLAKE |
| FORD | RESEARCH | 4 | JONES |
| MILLER | ACCOUNTING | 2 | CLARK |
+--------+------------+-------+--------+
14 rows in set (0.00 sec)