什么是连接查询
- 在实际开发中，一个业务对应多张表，一般需要联合查询取得最终结果
- 关系型数据库：大概意思就是里面的表是存在关系的
连接查询分类
- sql92
- sql99（新的）
在表的连接查询-笛卡尔积乘积现象
案例：找出每一个员工的部门名称，要求找出员工名，部门名
员工表
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 |
+--------+--------+
部门表
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)
联合查询，没有条件限制，那么查询结果是两个表的乘积
mysql> select ename,dname from emp,dept;
//别名：执行效率高
select e.ename,d.dname from emp e,dept d
+--------+------------+
| 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 (0.01 sec)
如何避免这个现象
- 加条件过滤
- 匹配次数还是56，但是显示14
- sql92 一般不用..........................
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)
内连接之等值连接：最大特点：条件是等量关系
案件：查询每个员工得部门编号，要求显示员工名和部门名
sql99 清晰，连接条件和where分离
-
select e.ename,d.dname from emp e join dept d on e.deptno = d.deptno;
-
a join b on 连接条件 where(过滤)..
mysql> select e.ename,d.dname from emp e join dept d 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 |
+--------+------------+
14 rows in set (0.00 sec)
内连接非等值连接：最大特点：条件不等
案例：找出每个员工得工资等级，显示 员工名 工资 工资等级
inner可以省略
mysql> select e.ename,e.sal,s.grade from emp e inner join salgrade s on e.sal between s.losal and s.hisal ;
+--------+---------+-------+
| 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 (0.03 sec)
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