MySQL資料庫學習筆記(三)

阿新 • • 發佈：2021-01-11

1、把查詢結果去除重複記錄

注意：原表資料不會修改，只是查詢結果去重

去重需要使用的關鍵字——distinct

mysql> select distinct job from emp;
+-----------+
| job       |
+-----------+
| CLERK     |
| SALESMAN  |
| MANAGER   |
| ANALYST   |
| PRESIDENT |
+-----------+
5 rows in set (0.00 sec)

//這樣編寫是有語法錯誤的；distinct只能出現在所有欄位最前方
mysql> select ename, distinct job from emp;

//distinct出現在job，deptno兩個欄位之前，表示兩個欄位聯合起來去重
mysql> select distinct job ,deptno from emp;
+-----------+--------+
| job       | deptno |
+-----------+--------+
| CLERK     |     20 |
| SALESMAN  |     30 |
| MANAGER   |     20 |
| MANAGER   |     30 |
| MANAGER   |     10 |
| ANALYST   |     20 |
| PRESIDENT |     10 |
| CLERK     |     30 |
| CLERK     |     10 |
+-----------+--------+
9 rows in set (0.00 sec)

統計一下工作崗位的數量

mysql> select count(distinct job) from emp;
+---------------------+
| count(distinct job) |
+---------------------+
|                   5 |
+---------------------+
1 row in set (0.01 sec)

2、連線查詢

2.1、什麼是連線查詢

從一張表中單獨查詢，稱為單獨查詢

emp表和dept表聯合起來查詢資料，從emp表中取員工名字，從dept表中取部門名字。這種跨表查詢，多張表聯合起來查詢資料，被稱為連線查詢。

2.2、連線查詢的分類

根據語法的年代分類：

SQL92：1992年的時候出現的語法

SQL99：1992年的時候出現的語法

根據表連線的方式分類：

內連線：等值連線

非等值連線

自連線

外連線：左外連線（左連線）

右外連線（右連線）

全連線：

2.3、當兩張表進行連線查詢時，沒有任何條件的限制會發生什麼現象？

案例：查詢每個員工所在部門名稱

mysql> select ename,deptno from emp;
+--------+--------+
| ename  | deptno |
+--------+--------+
| SMITH  |     20 |
| ALLEN  |     30 |
| WARD   |     30 |
| JONES  |     20 |
| MARTIN |     30 |
| BLAKE  |     30 |
| CLARK  |     10 |
| SCOTT  |     20 |
| KING   |     10 |
| TURNER |     30 |
| ADAMS  |     20 |
| JAMES  |     30 |
| FORD   |     20 |
| MILLER |     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)
//兩張表連線沒有任何限制：
mysql> select ename,dname from emp,dept;
+--------+------------+
| ename  | dname      |
+--------+------------+
| SMITH  | ACCOUNTING |
| SMITH  | RESEARCH   |
| SMITH  | SALES      |
| SMITH  | OPERATIONS |
| ALLEN  | ACCOUNTING |
| ALLEN  | RESEARCH   |
| ALLEN  | SALES      |
| ALLEN  | OPERATIONS |
| WARD   | ACCOUNTING |
| WARD   | RESEARCH   |
| WARD   | SALES      |
| WARD   | OPERATIONS |
.......
.......
56 rows in set (0.00 sec)  //14*4=56

當兩張表進行連線查詢，沒有任何條件限制的時候，最終查詢結果條數是兩張表條數的乘積，這種現象被稱為：“笛卡爾積現象”（數學現象）

2.4、怎麼避免笛卡爾積現象？

連線時加條件，滿足這個條件的記錄被篩選出來

select

ename,dname

from

emp,dept

where

emp.deptno = dept.deptno;//SQL92

mysql> select ename,dname from emp,dept where emp.deptno = dept.deptno;
+--------+------------+
| ename  | dname      |
+--------+------------+
| CLARK  | ACCOUNTING |
| KING   | ACCOUNTING |
| MILLER | ACCOUNTING |
| SMITH  | RESEARCH   |
| JONES  | RESEARCH   |
| SCOTT  | RESEARCH   |
| ADAMS  | RESEARCH   |
| FORD   | RESEARCH   |
| ALLEN  | SALES      |
| WARD   | SALES      |
| MARTIN | SALES      |
| BLAKE  | SALES      |
| TURNER | SALES      |
| JAMES  | SALES      |
+--------+------------+
14 rows in set (0.02 sec)

思考：最終查詢的結果的條數是14條，但是匹配的過程中，匹配的次數減少了嗎？

還是56次，只不過進行了四選一。次數沒有減少

注意：通過笛卡爾現象得出，表的連線次數越多，效率越低，儘量避免表的連線次數

//為提高效率，可以給表起別名
mysql> select e.ename,d.dname from emp e,dept d where e.deptno = d.deptno;  //92語法
+--------+------------+
| ename  | dname      |
+--------+------------+
| CLARK  | ACCOUNTING |
| KING   | ACCOUNTING |
| MILLER | ACCOUNTING |
| SMITH  | RESEARCH   |
| JONES  | RESEARCH   |
| SCOTT  | RESEARCH   |
| ADAMS  | RESEARCH   |
| FORD   | RESEARCH   |
| ALLEN  | SALES      |
| WARD   | SALES      |
| MARTIN | SALES      |
| BLAKE  | SALES      |
| TURNER | SALES      |
| JAMES  | SALES      |
+--------+------------+
14 rows in set (0.00 sec)

2.5、內連線之等值連線

案例：查詢每個員工所在部門的名稱，顯示員工名和部門名？

emp e 和 dept d 表進行連線，條件是：e.deptno = d.deptno

mysql> select e.ename,d.dname from emp e,dept d where e.deptno = d.deptno;//sql92缺點：結構不清晰，和後期進一步篩選的條件，都放到了where後面。
mysql> select e.ename,d.dname from emp e join dept d on e.deptno = d.deptno;//sql99優點：表連線的條件是獨立的，連線之後，如果還要進一步篩選，再往後繼續新增where
//語法結構：select...from...a inner（inner可以省略，可讀性好，一眼就能看出來是內連線） join b on a和b的連線條件 where 篩選條件
// 條件是等量關係，所以被稱為等值連線。
mysql> select e.ename,d.dname from emp e join dept d on e.deptno = d.deptno;
+--------+------------+
| ename  | dname      |
+--------+------------+
| CLARK  | ACCOUNTING |
| KING   | ACCOUNTING |
| MILLER | ACCOUNTING |
| SMITH  | RESEARCH   |
| JONES  | RESEARCH   |
| SCOTT  | RESEARCH   |
| ADAMS  | RESEARCH   |
| FORD   | RESEARCH   |
| ALLEN  | SALES      |
| WARD   | SALES      |
| MARTIN | SALES      |
| BLAKE  | SALES      |
| TURNER | SALES      |
| JAMES  | SALES      |
+--------+------------+
14 rows in set (0.00 sec)

2.6、內連線之非等值連線

案例：找出每個員工的薪資等級，要求顯示員工名、薪資、薪資等級？

mysql> select * from emp;
+-------+--------+-----------+------+------------+---------+---------+--------+
| EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM    | DEPTNO |
+-------+--------+-----------+------+------------+---------+---------+--------+
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |
|  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |
|  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |
|  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |
|  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |
|  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 |
|  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |
|  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |
|  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |
|  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |
|  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |
+-------+--------+-----------+------+------------+---------+---------+--------+
14 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,e.sal,s.grade 
       from emp e 
       join salgrade s 
       on e.sal 
       between s.losal and s.hisal;
       // 條件不是一個等量關係，稱為非等值連線。
+--------+---------+-------+
| ename  | sal     | grade |
+--------+---------+-------+
| SMITH  |  800.00 |     1 |
| ALLEN  | 1600.00 |     3 |
| WARD   | 1250.00 |     2 |
| JONES  | 2975.00 |     4 |
| MARTIN | 1250.00 |     2 |
| BLAKE  | 2850.00 |     4 |
| CLARK  | 2450.00 |     4 |
| SCOTT  | 3000.00 |     4 |
| KING   | 5000.00 |     5 |
| TURNER | 1500.00 |     3 |
| ADAMS  | 1100.00 |     1 |
| JAMES  |  950.00 |     1 |
| FORD   | 3000.00 |     4 |
| MILLER | 1300.00 |     2 |
+--------+---------+-------+
14 rows in set (0.00 sec)

2.7、內連線之自連線

案例：查詢員工的上級領導，要求顯示員工名和對應的領導名？

技巧：一張表看成兩張表。
emp a 員工表
mysql> select empno,ename,mgr from emp;
+-------+--------+------+
| empno | ename  | mgr  |
+-------+--------+------+
|  7369 | SMITH  | 7902 |
|  7499 | ALLEN  | 7698 |
|  7521 | WARD   | 7698 |
|  7566 | JONES  | 7839 |
|  7654 | MARTIN | 7698 |
|  7698 | BLAKE  | 7839 |
|  7782 | CLARK  | 7839 |
|  7788 | SCOTT  | 7566 |
|  7839 | KING   | NULL |
|  7844 | TURNER | 7698 |
|  7876 | ADAMS  | 7788 |
|  7900 | JAMES  | 7698 |
|  7902 | FORD   | 7566 |
|  7934 | MILLER | 7782 |
+-------+--------+------+
14 rows in set (0.00 sec)

emp b 領導表
+-------+--------+------+
| empno | ename  | mgr  |
+-------+--------+------+
|  7369 | SMITH  | 7902 |
|  7499 | ALLEN  | 7698 |
|  7521 | WARD   | 7698 |
|  7566 | JONES  | 7839 |
|  7654 | MARTIN | 7698 |
|  7698 | BLAKE  | 7839 |
|  7782 | CLARK  | 7839 |
|  7788 | SCOTT  | 7566 |
|  7839 | KING   | NULL |
|  7844 | TURNER | 7698 |
|  7876 | ADAMS  | 7788 |
|  7900 | JAMES  | 7698 |
|  7902 | FORD   | 7566 |
|  7934 | MILLER | 7782 |
+-------+--------+------+
select 
	a.ename as '員工名', b.ename as '領導名'
from
	emp a
join
	emp b
on
	a.mgr = b.empno; //員工的領導編號 = 領導的員工編號
+--------+--------+
| 員工名  | 領導名  |
+--------+--------+
| SMITH  | FORD   |
| ALLEN  | BLAKE  |
| WARD   | BLAKE  |
| JONES  | KING   |
| MARTIN | BLAKE  |
| BLAKE  | KING   |
| CLARK  | KING   |
| SCOTT  | JONES  |
| TURNER | BLAKE  |
| ADAMS  | SCOTT  |
| JAMES  | BLAKE  |
| FORD   | JONES  |
| MILLER | CLARK  |
+--------+--------+
13 rows in set (0.00 sec)
13條記錄，沒有KING。《內連線》

以上就是內連線中的：自連線，技巧：一張表看做兩張表。

2.8、外連線

mysql> select * from emp;
+-------+--------+-----------+------+------------+---------+---------+--------+
| EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM    | DEPTNO |
+-------+--------+-----------+------+------------+---------+---------+--------+
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |
|  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |
|  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |
|  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |
|  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |
|  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 |
|  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |
|  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |
|  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |
|  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |
|  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    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//內連線：（A和B連線，AB兩張表沒有主次關係。平等的。）
    -> e.ename,d.dname
    -> from
    -> emp e
    -> join
    -> dept d
    -> on
    -> e.deptno = d.deptno;
    //內連線的特點：完成能夠匹配上這個條件的資料查詢出來。
+--------+------------+
| ename  | dname      |
+--------+------------+
| CLARK  | ACCOUNTING |
| KING   | ACCOUNTING |
| MILLER | ACCOUNTING |
| SMITH  | RESEARCH   |
| JONES  | RESEARCH   |
| SCOTT  | RESEARCH   |
| ADAMS  | RESEARCH   |
| FORD   | RESEARCH   |
| ALLEN  | SALES      |
| WARD   | SALES      |
| MARTIN | SALES      |
| BLAKE  | SALES      |
| TURNER | SALES      |
| JAMES  | SALES      |
+--------+------------+
14 rows in set (0.00 sec)

外連線（右外連線）：
mysql> select
    -> e.ename,d.dname
    -> from
    -> emp e
    -> right （// outer是可以省略的，帶著可讀性強。）join
    -> dept d
    -> on
    -> e.deptno = d.deptno;
 //right代表什麼：表示將join關鍵字右邊的這張表看成主表，主要是為了將這張表的資料全部查詢出來，捎帶著關聯查詢左邊的表。
//在外連線當中，兩張表連線，產生了主次關係。
+--------+------------+
| ename  | dname      |
+--------+------------+
| CLARK  | ACCOUNTING |
| KING   | ACCOUNTING |
| MILLER | ACCOUNTING |
| SMITH  | RESEARCH   |
| JONES  | RESEARCH   |
| SCOTT  | RESEARCH   |
| ADAMS  | RESEARCH   |
| FORD   | RESEARCH   |
| ALLEN  | SALES      |
| WARD   | SALES      |
| MARTIN | SALES      |
| BLAKE  | SALES      |
| TURNER | SALES      |
| JAMES  | SALES      |
| NULL   | OPERATIONS |
+--------+------------+
15 rows in set (0.01 sec)

外連線（左外連線）：
mysql> select
    -> e.ename,d.dname
    -> from
    -> dept d
    -> left（// outer是可以省略的，帶著可讀性強。） join
    -> emp e
    -> on
    -> e.deptno = d.deptno;
+--------+------------+
| ename  | dname      |
+--------+------------+
| CLARK  | ACCOUNTING |
| KING   | ACCOUNTING |
| MILLER | ACCOUNTING |
| SMITH  | RESEARCH   |
| JONES  | RESEARCH   |
| SCOTT  | RESEARCH   |
| ADAMS  | RESEARCH   |
| FORD   | RESEARCH   |
| ALLEN  | SALES      |
| WARD   | SALES      |
| MARTIN | SALES      |
| BLAKE  | SALES      |
| TURNER | SALES      |
| JAMES  | SALES      |
| NULL   | OPERATIONS |
+--------+------------+
15 rows in set (0.00 sec)

帶有right的是右外連線，又叫做右連線。
帶有left的是左外連線，又叫做左連線。
任何一個右連線都有左連線的寫法。
任何一個左連線都有右連線的寫法。