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尔积进行筛选，之前谈到的查询都是内连接。

语法：select 字段 from 表1 inner join 表2 on 连接条件 and 其他条件;

• 显示SMITH的名字和部门名字

  # 方法1
  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)
  
  # 方法2
  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)

外连接

左外连接

左侧的表完全显示，右侧的表不做要求。

语法：select 字段名 from 表名1 left join 表名2 on 连接条件;

• 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

  # 首先建两张表
  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);
  
  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> 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。

语法：select 字段 from 表名1 right join 表名2 on 连接条件;

• 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

  mysql> select dname,ename,dept.deptno from dept left join emp on dept.deptno=emp.deptno;
  +------------+--------+--------+
  | dname      | ename  | deptno |
  +------------+--------+--------+
  | ACCOUNTING | MILLER |     10 |
  | ACCOUNTING | KING   |     10 |
  | ACCOUNTING | CLARK  |     10 |
  | RESEARCH   | FORD   |     20 |
  | RESEARCH   | ADAMS  |     20 |
  | RESEARCH   | SCOTT  |     20 |
  | RESEARCH   | JONES  |     20 |
  | RESEARCH   | SMITH  |     20 |
  | SALES      | JAMES  |     30 |
  | SALES      | TURNER |     30 |
  | SALES      | BLAKE  |     30 |
  | SALES      | MARTIN |     30 |
  | SALES      | WARD   |     30 |
  | SALES      | ALLEN  |     30 |
  | OPERATIONS | NULL   |     40 |
  +------------+--------+--------+
  15 rows in set (0.00 sec)