内外连接【MySQL】

表的内外连接

显示SMITH的名字和部门名称

内连接

第一种写法，用笛卡尔积

mysql> select emp.ename,dept.dname from emp,dept where emp.deptno=dept.deptno ;
+--------+------------+
| ename  | dname      |
+--------+------------+
| SMITH  | RESEARCH   |
| ALLEN  | SALES      |
| WARD   | SALES      |
| JONES  | RESEARCH   |
| MARTIN | SALES      |
| BLAKE  | SALES      |
| CLARK  | ACCOUNTING |
| SCOTT  | RESEARCH   |
| KING   | ACCOUNTING |
| TURNER | SALES      |
| ADAMS  | RESEARCH   |
| JAMES  | SALES      |
| FORD   | RESEARCH   |
| MILLER | ACCOUNTING |
+--------+------------+
14 rows in set (0.00 sec)

内连接

mysql> select ename,dname  from emp inner join dept on emp.deptno=dept.deptno ;
+--------+------------+
| ename  | dname      |
+--------+------------+
| SMITH  | RESEARCH   |
| ALLEN  | SALES      |
| WARD   | SALES      |
| JONES  | RESEARCH   |
| MARTIN | SALES      |
| BLAKE  | SALES      |
| CLARK  | ACCOUNTING |
| SCOTT  | RESEARCH   |
| KING   | ACCOUNTING |
| TURNER | SALES      |
| ADAMS  | RESEARCH   |
| JAMES  | SALES      |
| FORD   | RESEARCH   |
| MILLER | ACCOUNTING |
+--------+------------+
14 rows in set (0.00 sec)mysql> select ename,dname  from emp inner join dept on emp.deptno=dept.deptno where ename='SMITH';
+-------+----------+
| ename | dname    |
+-------+----------+
| SMITH | RESEARCH |
+-------+----------+
1 row in set (0.00 sec)

外连接

create table stu (id int, name varchar(30)); -- 学生表
insert into stu values(1,'jack'),(2,'tom'),(3,'kity'),(4,'nono');
create table exam (id int, grade int); -- 成绩表
insert into exam values(1, 56),(2,76),(11, 8);

例如：查询所有学生的成绩，如果这个学生没有成绩，也要将学生的个人信息显示出来

左外连接：如果联合查询，左侧的表完全显示，右侧的表按条件拼接，条件满足，拼接上，若条件不满足，拼接NULL

mysql> select * from stu left join exam on stu.id=exam.id;
+------+------+------+-------+
| id   | name | id   | grade |
+------+------+------+-------+
|    1 | jack |    1 |    56 |
|    2 | tom  |    2 |    76 |
|    3 | kity | NULL |  NULL |
|    4 | nono | NULL |  NULL |
+------+------+------+-------+
4 rows in set (0.00 sec)

右外连接
如果联合查询，右侧的表完全显示我们就说是右外连接

例如：对stu表和exam表联合查询，把所有的成绩都显示出来，即使这个成绩没有学生与它对应，也要
显示出来

mysql> select * from stu right join exam on stu.id=exam.id;
+------+------+------+-------+
| id   | name | id   | grade |
+------+------+------+-------+
|    1 | jack |    1 |    56 |
|    2 | tom  |    2 |    76 |
| NULL | NULL |   11 |     8 |
+------+------+------+-------+
3 rows in set (0.00 sec)

列出部门名称和这些部门的员工信息，同时列出没有员工的部门

1、左外连接

mysql> select * from dept left join  emp on dept.deptno=emp.deptno;
+--------+------------+----------+--------+--------+-----------+------+---------------------+---------+---------+--------+
| deptno | dname      | loc      | empno  | ename  | job       | mgr  | hiredate            | sal     | comm    | deptno |
+--------+------------+----------+--------+--------+-----------+------+---------------------+---------+---------+--------+
|     20 | RESEARCH   | DALLAS   | 007369 | SMITH  | CLERK     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |
|     30 | SALES      | CHICAGO  | 007499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |
|     30 | SALES      | CHICAGO  | 007521 | WARD   | SALESMAN  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |
|     20 | RESEARCH   | DALLAS   | 007566 | JONES  | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |
|     30 | SALES      | CHICAGO  | 007654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |
|     30 | SALES      | CHICAGO  | 007698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |
|     10 | ACCOUNTING | NEW YORK | 007782 | CLARK  | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |
|     20 | RESEARCH   | DALLAS   | 007788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 |    NULL |     20 |
|     10 | ACCOUNTING | NEW YORK | 007839 | KING   | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |
|     30 | SALES      | CHICAGO  | 007844 | TURNER | SALESMAN  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |
|     20 | RESEARCH   | DALLAS   | 007876 | ADAMS  | CLERK     | 7788 | 1987-05-23 00:00:00 | 1100.00 |    NULL |     20 |
|     30 | SALES      | CHICAGO  | 007900 | JAMES  | CLERK     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |
|     20 | RESEARCH   | DALLAS   | 007902 | FORD   | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |
|     10 | ACCOUNTING | NEW YORK | 007934 | MILLER | CLERK     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |
|     40 | OPERATIONS | BOSTON   |   NULL | NULL   | NULL      | NULL | NULL                |    NULL |    NULL |   NULL |
+--------+------------+----------+--------+--------+-----------+------+---------------------+---------+---------+--------+
15 rows in set (0.01 sec)mysql> select * from dept left join  emp on dept.deptno=emp.deptno  order by dept.deptno asc;
+--------+------------+----------+--------+--------+-----------+------+---------------------+---------+---------+--------+
| deptno | dname      | loc      | empno  | ename  | job       | mgr  | hiredate            | sal     | comm    | deptno |
+--------+------------+----------+--------+--------+-----------+------+---------------------+---------+---------+--------+
|     10 | ACCOUNTING | NEW YORK | 007782 | CLARK  | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |
|     10 | ACCOUNTING | NEW YORK | 007839 | KING   | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |
|     10 | ACCOUNTING | NEW YORK | 007934 | MILLER | CLERK     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |
|     20 | RESEARCH   | DALLAS   | 007902 | FORD   | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |
|     20 | RESEARCH   | DALLAS   | 007788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 |    NULL |     20 |
|     20 | RESEARCH   | DALLAS   | 007876 | ADAMS  | CLERK     | 7788 | 1987-05-23 00:00:00 | 1100.00 |    NULL |     20 |
|     20 | RESEARCH   | DALLAS   | 007369 | SMITH  | CLERK     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |
|     20 | RESEARCH   | DALLAS   | 007566 | JONES  | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |
|     30 | SALES      | CHICAGO  | 007844 | TURNER | SALESMAN  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |
|     30 | SALES      | CHICAGO  | 007521 | WARD   | SALESMAN  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |
|     30 | SALES      | CHICAGO  | 007698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |
|     30 | SALES      | CHICAGO  | 007900 | JAMES  | CLERK     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |
|     30 | SALES      | CHICAGO  | 007499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |
|     30 | SALES      | CHICAGO  | 007654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |
|     40 | OPERATIONS | BOSTON   |   NULL | NULL   | NULL      | NULL | NULL                |    NULL |    NULL |   NULL |
+--------+------------+----------+--------+--------+-----------+------+---------------------+---------+---------+--------+
15 rows in set (0.00 sec)mysql> select  dname,ename,dept.deptno  from dept left join  emp on dept.deptno=emp.deptno  order by dept.deptno asc;
+------------+--------+--------+
| dname      | ename  | deptno |
+------------+--------+--------+
| ACCOUNTING | KING   |     10 |
| ACCOUNTING | CLARK  |     10 |
| ACCOUNTING | MILLER |     10 |
| RESEARCH   | ADAMS  |     20 |
| RESEARCH   | SCOTT  |     20 |
| RESEARCH   | FORD   |     20 |
| RESEARCH   | JONES  |     20 |
| RESEARCH   | SMITH  |     20 |
| SALES      | MARTIN |     30 |
| SALES      | ALLEN  |     30 |
| SALES      | JAMES  |     30 |
| SALES      | BLAKE  |     30 |
| SALES      | WARD   |     30 |
| SALES      | TURNER |     30 |
| OPERATIONS | NULL   |     40 |
+------------+--------+--------+
15 rows in set (0.00 sec)