表的内连和外连（重点）
表的连接分为内连和外连
1. 内连接
内连接实际上就是利用where子句对两种表形成的笛卡儿积进行筛选，我们前面学习的查询都是内连接，也是在开发过程中使用的最多的连接查询。
语法：
select 字段 from 表1 inner join 表2 on 连接条件 and 其他条件；
备注：前面学习的都是内连接
案例：
显示SMITH的名字和部门名称
-- 用前面的写法
mysql> select ename,dname from emp,dept where emp.deptno = dept.deptno and ename = "SMITH";
+-------+----------+
| ename | dname |
+-------+----------+
| SMITH | RESEARCH |
+-------+----------+
1 row in set (0.00 sec)
-- 用标准的内连接写法
mysql> select ename,dname from emp inner join dept on emp.deptno = dept.deptno and ename = "SMITH";
+-------+----------+
| ename | dname |
+-------+----------+
| SMITH | RESEARCH |
+-------+----------+
1 row in set (0.00 sec)
2. 外连接
外连接分为左外连接和右外连接
2.1 左外连接
如果联合查询，左侧的表完全显示我们就说是左外连接。
语法：
select 字段名 from 表名1 left join 表名2 on 连接条件
案例：
mysql> -- 建两张表
mysql> create table stu (id int, name varchar(30)); -- 学生表
Query OK, 0 rows affected (0.01 sec)
mysql> insert into stu values(1,'jack'),(2,'tom'),(3,'kity'),(4,'nono');
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0
mysql> create table exam (id int, grade int); -- 成绩表
Query OK, 0 rows affected (0.01 sec)
mysql> insert into exam values(1, 56),(2,76),(11, 8);
Query OK, 3 rows affected (0.00 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> select * from stu;
+------+------+
| id | name |
+------+------+
| 1 | jack |
| 2 | tom |
| 3 | kity |
| 4 | nono |
+------+------+
4 rows in set (0.00 sec)
mysql> select * from exam;
+------+-------+
| id | grade |
+------+-------+
| 1 | 56 |
| 2 | 76 |
| 11 | 8 |
+------+-------+
3 rows in set (0.00 sec)
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
mysql> -- 当左边表和右边表没有匹配时，也会显示左边表的数据
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)
-- 上述情况也就是说kity和nono并未参加考试所以没有考试成绩
2.1 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。
语法：
select 字段 from 表名1 right join 表名2 on 连接条件；
案例：
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)
练习：
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
-- 方法一：
mysql> select d.dname, e.* from dept d left join emp e on d.deptno=e.deptno;
+------------+--------+--------+-----------+------+---------------------+---------+---------+--------+
| dname | empno | ename | job | mgr | hiredate | sal | comm | deptno |
+------------+--------+--------+-----------+------+---------------------+---------+---------+--------+
| RESEARCH | 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 |
| SALES | 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 |
| SALES | 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 |
| RESEARCH | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 |
| SALES | 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 |
| SALES | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 |
| ACCOUNTING | 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 |
| RESEARCH | 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 |
| ACCOUNTING | 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 |
| SALES | 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 |
| RESEARCH | 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 |
| SALES | 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 |
| RESEARCH | 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 |
| ACCOUNTING | 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 |
| OPERATIONS | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL |
+------------+--------+--------+-----------+------+---------------------+---------+---------+--------+
15 rows in set (0.00 sec)
-- 方法二：
mysql> select d.dname, e.* from emp e right join dept d on d.deptno=e.deptno;
+------------+--------+--------+-----------+------+---------------------+---------+---------+--------+
| dname | empno | ename | job | mgr | hiredate | sal | comm | deptno |
+------------+--------+--------+-----------+------+---------------------+---------+---------+--------+
| RESEARCH | 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 |
| SALES | 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 |
| SALES | 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 |
| RESEARCH | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 |
| SALES | 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 |
| SALES | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 |
| ACCOUNTING | 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 |
| RESEARCH | 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 |
| ACCOUNTING | 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 |
| SALES | 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 |
| RESEARCH | 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 |
| SALES | 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 |
| RESEARCH | 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 |
| ACCOUNTING | 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 |
| OPERATIONS | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL |
+------------+--------+--------+-----------+------+---------------------+---------+---------+--------+
15 rows in set (0.00 sec)
其实也就是多了这条数据
