表的连接分类
表的连接主要分为内连接和外连接两种类型。
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，这是我们前面学习的主要查询方式，也是开发过程中使用最多的连接查询。
我们除了可以使用" , "来隔离表，我们还可以使用下面的语法：和我们之前笛卡尔积，后面使用where条件筛选是一样的效果的！只不过下面的写法是比较标准的！
sql
SELECT 字段 FROM 表1 INNER JOIN 表2 ON 连接条件 AND 其他条件；前面学习的大部分查询都属于内连接。
案例：显示SMITH的名字和部门名称
传统写法：
sql
SELECT ename, dname FROM EMP, DEPT WHERE EMP.deptno=DEPT.deptno AND ename='SMITH';
sql
mysql> select emp.ename, dept.dname from emp, dept where emp.deptno=dept.deptno and emp.ename='SMITH';
+-------+----------+
| ename | dname |
+-------+----------+
| SMITH | RESEARCH |
+-------+----------+
1 row in set (0.00 sec)标准内连接写法：
sql
SELECT ename, dname FROM EMP INNER JOIN DEPT ON EMP.deptno=DEPT.deptno AND ename='SMITH';
sql
mysql> select emp.ename, dept.dname from emp inner join dept on emp.deptno=dept.deptno and emp.ename='SMITH';
+-------+----------+
| ename | dname |
+-------+----------+
| SMITH | RESEARCH |
+-------+----------+
1 row in set (0.00 sec)外连接
外连接分为左外连接和右外连接。
为主的在哪一边，基本就是使用那一边的连接方式！--- 后面的练习部分体现！！！
左外连接
如果联合查询时，左侧的表完全显示（左侧的信息不要做任何的过滤和筛选，完全保留）（即使右侧没有匹配记录），我们就称为左外连接。
仅仅将 inner join 改成了 left join
sql
SELECT 字段名 FROM 表名1 LEFT JOIN 表名2 ON 连接条件创建测试表：
sql
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，如果学生没有成绩也要显示个人信息：
sql
SELECT * FROM stu LEFT JOIN exam ON stu.id=exam.id;
sql
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)这个查询会显示左边表的所有记录，即使右边表没有匹配的记录。
右外连接
如果联合查询时，右侧的表完全显示（即使左侧没有匹配记录），我们就称为右外连接。
sql
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，显示所有成绩，即使没有对应学生：
sql
SELECT * FROM stu RIGHT JOIN exam ON stu.id=exam.id;
sql
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（使用左连接）：
sql
SELECT d.dname, e.* FROM dept d LEFT JOIN emp e ON d.deptno=e.deptno;
sql
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 |
+------------+--------+--------+-----------+------+---------------------+---------+---------+--------+
| ACCOUNTING | 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 |
| ACCOUNTING | 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 |
| ACCOUNTING | 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 |
| RESEARCH | 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 |
| RESEARCH | 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 |
| RESEARCH | 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 |
| RESEARCH | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 |
| RESEARCH | 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 |
| SALES | 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 |
| SALES | 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 |
| SALES | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 |
| SALES | 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 |
| SALES | 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 |
| SALES | 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 |
| OPERATIONS | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL |
+------------+--------+--------+-----------+------+---------------------+---------+---------+--------+
15 rows in set (0.00 sec)方法二（使用右连接）：
sql
SELECT d.dname, e.* FROM emp e RIGHT JOIN dept d ON d.deptno=e.deptno;
sql
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 |
+------------+--------+--------+-----------+------+---------------------+---------+---------+--------+
| ACCOUNTING | 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 |
| ACCOUNTING | 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 |
| ACCOUNTING | 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 |
| RESEARCH | 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 |
| RESEARCH | 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 |
| RESEARCH | 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 |
| RESEARCH | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 |
| RESEARCH | 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 |
| SALES | 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 |
| SALES | 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 |
| SALES | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 |
| SALES | 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 |
| SALES | 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 |
| SALES | 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 |
| OPERATIONS | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL |
+------------+--------+--------+-----------+------+---------------------+---------+---------+--------+
15 rows in set (0.00 sec)