九、连接查询
9.1 什么是连接查询
从一张表中单独查询，称为单表查询。
emp表和dept表联合起来查询数据，从emp表中取员工名字，从dept表中取部门名字。这种跨表查询，多张表联合起来查询数据，被称为连接查询。
9.2 连接查询的分类
根据语法的年代分类：
- SQL92：1992年的时候出现的语法
- SQL99：1999年的时候出现的语法
根据表连接的方式分类：
- 内连接：（A和B连接，AB两张表没有主次关系~平等的）
- 等值连接
- 非等值连接
- 自连接
- 外连接：
- 左外连接（左连接）
- 右外连接（右连接）
- 全连接
9.3 没有任何条件限制的两张表进行连接查询
当两张表进行连接查询，没有任何条件限制的时候，最终查询结果条数是两张表条数的乘积，这种现象被称为：笛卡尔积现象 。
查询每个员工所在部门名称：
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)
# 两张表连接没有任何条件限制：
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 |
| JONES | ACCOUNTING |
| JONES | RESEARCH |
| JONES | SALES |
| JONES | OPERATIONS |
| MARTIN | ACCOUNTING |
| MARTIN | RESEARCH |
| MARTIN | SALES |
| MARTIN | OPERATIONS |
| BLAKE | ACCOUNTING |
| BLAKE | RESEARCH |
| BLAKE | SALES |
| BLAKE | OPERATIONS |
| CLARK | ACCOUNTING |
| CLARK | RESEARCH |
| CLARK | SALES |
| CLARK | OPERATIONS |
| SCOTT | ACCOUNTING |
| SCOTT | RESEARCH |
| SCOTT | SALES |
| SCOTT | OPERATIONS |
| KING | ACCOUNTING |
| KING | RESEARCH |
| KING | SALES |
| KING | OPERATIONS |
| TURNER | ACCOUNTING |
| TURNER | RESEARCH |
| TURNER | SALES |
| TURNER | OPERATIONS |
| ADAMS | ACCOUNTING |
| ADAMS | RESEARCH |
| ADAMS | SALES |
| ADAMS | OPERATIONS |
| JAMES | ACCOUNTING |
| JAMES | RESEARCH |
| JAMES | SALES |
| JAMES | OPERATIONS |
| FORD | ACCOUNTING |
| FORD | RESEARCH |
| FORD | SALES |
| FORD | OPERATIONS |
| MILLER | ACCOUNTING |
| MILLER | RESEARCH |
| MILLER | SALES |
| MILLER | OPERATIONS |
+--------+------------+
56 rows in set (0.00 sec)
14 * 4 = 56
9.4 如何避免笛卡尔积现象？
在连接时加入条件，满足这个条件的记录被筛选出来。
mysql> select ename, 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)
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)
# 给表起别名，很重要-----效率问题
mysql> select e.ename, d.dname
-> from emp e, dept d
-> where e.deptno = d.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)
通过笛卡尔积现象得出，表的连接次数越多效率越低，尽量避免表的连接次数。
9.5 内连接–等值连接
SQL92语法的缺点：结构不清晰，表的连接条件和后期进一步筛选的条件都放到了 where 后面。
SQL99的优点：表连接的条件是独立的，连接之后，如果还需要进一步筛选，再往后继续添加where
# SQL99语法格式：
select ...
from a
join b
on a和b的连接条件
where 筛选条件
查询每个员工所在部门名称，显示员工名和部门名：
# emp e和dept d表进行连接。条件是：e.deptno = d.deptno
# SQL92语法
mysql> select e.ename, d.ename
-> from emp e, dept d
-> where e.deptno = d.deptno;
ERROR 1054 (42S22): Unknown column 'd.ename' in 'field list'
mysql> select e.ename, d.dname
-> from emp e, dept d
-> where e.deptno = d.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)
# SQL99语法：
mysql> select e.ename, d.dname
-> from emp e
-> join dept d # inner可以省略（带着inner可读性更好！inner join）
-> on e.deptno = d.deptno; # 条件时等量关系，所以被称为等值连接，完成能够匹配上这个条件的数据查询出来
+--------+------------+
| 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)
9.6 内连接–非等值连接
找出每个员工的薪资等级，要求显示员工名、薪资、薪资等级：
# 查看一下emp和salgrade表的结构
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 (0.01 sec)
mysql> select e.ename, e.sal, s.grade
-> from emp e
-> join salgrade s
-> on e.sal between s.losal and s.hisal; # 条件不是一个等量关系，称为非等值连接
+--------+---------+-------+
| 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)
9.7 内连接–自连接
技巧：一张表看作两张表。
查询员工的上级领导，要求显示员工名和对应的领导名：
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)
# 技巧：将一张表看作两张表
# emp a 员工表
+-------+--------+------+
| 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 |
+-------+--------+------+
14 rows in set (0.00 sec)
# emp b 领导表
+-------+--------+------+
| 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 |
+-------+--------+------+
14 rows in set (0.00 sec)
mysql> select a.ename as '员工名', b.ename as '领导名'
-> from emp a
-> join emp b
-> on a.mgr = b.empno; # 员工的领导编号 = 领导的员工编号
+-----------+-----------+
| 员工名 | 领导名 |
+-----------+-----------+
| 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条记录，没有KING连接《内连接》
9.8 外连接–右外连接
right的含义：表示将join关键字右边的这张表看作主表，主要是为了将这张表的数据全部查询出来，捎带着关联查询左边的表。
在外连接中，两张表的连接产生了主次关系。
mysql> select e.ename, d.dname
-> from emp e
-> right outer join dept d # outer可以省略，带着可读性更强
-> on e.deptno = d.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 |
| NULL | OPERATIONS |
+--------+------------+
15 rows in set (0.00 sec)
9.9 外连接–左外连接
mysql> select e.ename, d.dname
-> from dept d
-> left outer join emp e # outer可以省略
-> on e.deptno = d.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 |
| NULL | OPERATIONS |
+--------+------------+
15 rows in set (0.00 sec)
带有right的是右外连接，又叫做右连接
带有left的是做外外连接，又叫做左连接任何一个右连接都有左连接的写法
任何一个左连接都有右连接的写法
思考：外连接的查询结果条数一定是 >= 内连接的查询结果条数吗？
===》正确！
9.10 案例
查询每个员工的上级领导，要求显示所有员工的名字和领导名：
mysql> select a.ename as '员工名', b.ename as '领导名'
-> from emp a
-> left join emp b
-> on a.mgr = b.empno;
+-----------+-----------+
| 员工名 | 领导名 |
+-----------+-----------+
| SMITH | FORD |
| ALLEN | BLAKE |
| WARD | BLAKE |
| JONES | KING |
| MARTIN | BLAKE |
| BLAKE | KING |
| CLARK | KING |
| SCOTT | JONES |
| KING | NULL |
| TURNER | BLAKE |
| ADAMS | SCOTT |
| JAMES | BLAKE |
| FORD | JONES |
| MILLER | CLARK |
+-----------+-----------+
14 rows in set (0.00 sec)
9.11 多张表的连接
# 语法：
select ...
from a
join b
on a和b的连接条件
join c
on a和c的连接条件
right join d
on a和d的连接条件
# 一条SQL中内连接和外连接可以混合----可以都出现
找出每个员工的部门名称以及工资等级，要求显示员工名、部门名、薪资、薪资等级：
mysql> select e.ename, d.dname, e.sal, s.grade
-> from emp e
-> join dept d
-> on e.deptno = d.deptno
-> join salgrade s
-> on e.sal between s.losal and s.hisal;
+--------+------------+---------+-------+
| ename | dname | sal | grade |
+--------+------------+---------+-------+
| SMITH | RESEARCH | 800.00 | 1 |
| ALLEN | SALES | 1600.00 | 3 |
| WARD | SALES | 1250.00 | 2 |
| JONES | RESEARCH | 2975.00 | 4 |
| MARTIN | SALES | 1250.00 | 2 |
| BLAKE | SALES | 2850.00 | 4 |
| CLARK | ACCOUNTING | 2450.00 | 4 |
| SCOTT | RESEARCH | 3000.00 | 4 |
| KING | ACCOUNTING | 5000.00 | 5 |
| TURNER | SALES | 1500.00 | 3 |
| ADAMS | RESEARCH | 1100.00 | 1 |
| JAMES | SALES | 950.00 | 1 |
| FORD | RESEARCH | 3000.00 | 4 |
| MILLER | ACCOUNTING | 1300.00 | 2 |
+--------+------------+---------+-------+
14 rows in set (0.00 sec)
找出每个员工的部门名称以及工资等级，还有上级领导，要求显示员工名、领导名、部门名、薪资、薪资等级：
mysql> select e.ename, d.dname, l.ename, e.sal, s.grade
-> from emp e
-> join dept d
-> on e.deptno = d.deptno
-> join salgrade s
-> on e.sal between s.losal and s.hisal
-> left join emp l
-> on e.mgr = l.empno;
+--------+------------+-------+---------+-------+
| ename | dname | ename | sal | grade |
+--------+------------+-------+---------+-------+
| SMITH | RESEARCH | FORD | 800.00 | 1 |
| ADAMS | RESEARCH | SCOTT | 1100.00 | 1 |
| JAMES | SALES | BLAKE | 950.00 | 1 |
| WARD | SALES | BLAKE | 1250.00 | 2 |
| MARTIN | SALES | BLAKE | 1250.00 | 2 |
| MILLER | ACCOUNTING | CLARK | 1300.00 | 2 |
| ALLEN | SALES | BLAKE | 1600.00 | 3 |
| TURNER | SALES | BLAKE | 1500.00 | 3 |
| JONES | RESEARCH | KING | 2975.00 | 4 |
| BLAKE | SALES | KING | 2850.00 | 4 |
| CLARK | ACCOUNTING | KING | 2450.00 | 4 |
| SCOTT | RESEARCH | JONES | 3000.00 | 4 |
| FORD | RESEARCH | JONES | 3000.00 | 4 |
| KING | ACCOUNTING | NULL | 5000.00 | 5 |
+--------+------------+-------+---------+-------+
14 rows in set (0.00 sec)