多表联查
1、把查询结果去除重复记录【distinct】
注意：原表数据不会被修改，只是查询结果去重。
去重需要使用一个关键字：distinct
mysql> select distinct job from emp;
+-----------+
| job |
+-----------+
| CLERK |
| SALESMAN |
| MANAGER |
| ANALYST |
| PRESIDENT |
+-----------+
// 这样编写是错误的，语法错误。
// distinct只能出现在所有字段的最前方。
mysql> select ename,distinct job from emp;
// distinct出现在job,deptno两个字段之前，表示两个字段联合起来去重。
mysql> select distinct job,deptno from emp;
+-----------+--------+
| job | deptno |
+-----------+--------+
| CLERK | 20 |
| SALESMAN | 30 |
| MANAGER | 20 |
| MANAGER | 30 |
| MANAGER | 10 |
| ANALYST | 20 |
| PRESIDENT | 10 |
| CLERK | 30 |
| CLERK | 10 |
+-----------+--------+
统计一下工作岗位的数量？
select count(distinct job) from emp;
+---------------------+
| count(distinct job) |
+---------------------+
| 5 |
+---------------------+2、连接查询
2.1、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。
2.2、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
我们这里重点学习SQL99.(这个过程中简单演示一个SQL92的例子)
根据表连接的方式分类：
内连接：
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（不讲）2.3、当两张表进行连接查询时，没有任何条件的限制会发生什么现象？
案例：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 |
+--------+--------+
mysql> select * from dept;
+--------+------------+----------+
| DEPTNO | DNAME | LOC |
+--------+------------+----------+
| 10 | ACCOUNTING | NEW YORK |
| 20 | RESEARCH | DALLAS |
| 30 | SALES | CHICAGO |
| 40 | OPERATIONS | BOSTON |
+--------+------------+----------+
两张表连接没有任何条件限制：
select ename,dname from emp, dept;
+--------+------------+
| ename | dname |
+--------+------------+
| SMITH | ACCOUNTING |
| SMITH | RESEARCH |
| SMITH | SALES |
| SMITH | OPERATIONS |
| ALLEN | ACCOUNTING |
| ALLEN | RESEARCH |
| ALLEN | SALES |
| ALLEN | OPERATIONS |
...
56 rows in set (0.00 sec)
14 * 4 = 56
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。（笛卡尔发现的，这是
一个数学现象。）2.4、怎么避免笛卡尔积现象？
连接时加条件，满足这个条件的记录被筛选出来！
select
ename,dname
from
emp, dept
where
emp.deptno = dept.deptno;
select
emp.ename,dept.dname
from
emp, dept
where
emp.deptno = dept.deptno;
// 表起别名。很重要。效率问题。
select
e.ename,d.dname
from
emp e, dept d
where
e.deptno = d.deptno; //SQL92语法。
+--------+------------+
| ename | dname |
+--------+------------+
| CLARK | ACCOUNTING |
| KING | ACCOUNTING |
| MILLER | ACCOUNTING |
| SMITH | RESEARCH |
| JONES | RESEARCH |
| SCOTT | RESEARCH |
| ADAMS | RESEARCH |
| FORD | RESEARCH |
| ALLEN | SALES |
| WARD | SALES |
| MARTIN | SALES |
| BLAKE | SALES |
| TURNER | SALES |
| JAMES | SALES |
+--------+------------+
思考：最终查询的结果条数是14条，但是匹配的过程中，匹配的次数减少了吗？
还是56次，只不过进行了四选一。次数没有减少。
注意：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。2.5、内连接之等值连接。
案例：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：
select
e.ename,d.dname
from
emp e, dept d
where
e.deptno = d.deptno;
sql92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语法：
select
e.ename,d.dname
from
emp e
join
dept d
on
e.deptno = d.deptno;
//inner可以省略（带着inner可读性更好！！！一眼就能看出来是内连接）
select
e.ename,d.dname
from
emp e
inner join
dept d
on
e.deptno = d.deptno; // 条件是等量关系，所以被称为等值连接。
sql99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：
select
...
from
a
join
b
on
a和b的连接条件
where
筛选条件
2.6、内连接之非等值连接
案例：找出每个员工的薪资等级，要求显示员工名、薪资、薪资等级？
mysql> select * from emp; e
+-------+--------+-----------+------+------------+---------+---------+--------+
| 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 |
....
mysql> select * from salgrade; s
+-------+-------+-------+
| GRADE | LOSAL | HISAL |
+-------+-------+-------+
| 1 | 700 | 1200 |
| 2 | 1201 | 1400 |
| 3 | 1401 | 2000 |
| 4 | 2001 | 3000 |
| 5 | 3001 | 9999 |
+-------+-------+-------+
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。
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 |
+--------+---------+-------+
2.7、内连接之自连接
案例：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 |
+-------+--------+------+
技巧：一张表看成两张表。
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 |
+-------+--------+------+
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 |
+-------+--------+------+
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条记录，没有KING。《内连接》
以上就是内连接中的：自连接，技巧：一张表看做两张表。
2.8、外连接
mysql> select * from emp; e
+-------+--------+-----------+------+------------+---------+---------+--------+
| 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 |
+-------+--------+-----------+------+------------+---------+---------+--------+
mysql> select * from dept; d
+--------+------------+----------+
| DEPTNO | DNAME | LOC |
+--------+------------+----------+
| 10 | ACCOUNTING | NEW YORK |
| 20 | RESEARCH | DALLAS |
| 30 | SALES | CHICAGO |
| 40 | OPERATIONS | BOSTON |
+--------+------------+----------+
内连接：（A和B连接，AB两张表没有主次关系。平等的。）
select
e.ename,d.dname
from
emp e
join
dept d
on
e.deptno = d.deptno; //内连接的特点：完成能够匹配上这个条件的数据查询出来。
+--------+------------+
| ename | dname |
+--------+------------+
| CLARK | ACCOUNTING |
| KING | ACCOUNTING |
| MILLER | ACCOUNTING |
| SMITH | RESEARCH |
| JONES | RESEARCH |
| SCOTT | RESEARCH |
| ADAMS | RESEARCH |
| FORD | RESEARCH |
| ALLEN | SALES |
| WARD | SALES |
| MARTIN | SALES |
| BLAKE | SALES |
| TURNER | SALES |
| JAMES | SALES |
+--------+------------+
外连接（右外连接）：
select
e.ename,d.dname
from
emp e
right join
dept d
on
e.deptno = d.deptno;
// outer是可以省略的，带着可读性强。
select
e.ename,d.dname
from
emp e
right outer join
dept d
on
e.deptno = d.deptno;
right代表什么：表示将join关键字右边的这张表看成主表，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。
在外连接当中，两张表连接，产生了主次关系。
外连接（左外连接）：
select
e.ename,d.dname
from
dept d
left join
emp e
on
e.deptno = d.deptno;
// outer是可以省略的，带着可读性强。
select
e.ename,d.dname
from
dept d
left outer join
emp e
on
e.deptno = d.deptno;
带有right的是右外连接，又叫做右连接。
带有left的是左外连接，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。
+--------+------------+
| ename | dname |
+--------+------------+
| CLARK | ACCOUNTING |
| KING | ACCOUNTING |
| MILLER | ACCOUNTING |
| SMITH | RESEARCH |
| JONES | RESEARCH |
| SCOTT | RESEARCH |
| ADAMS | RESEARCH |
| FORD | RESEARCH |
| ALLEN | SALES |
| WARD | SALES |
| MARTIN | SALES |
| BLAKE | SALES |
| TURNER | SALES |
| JAMES | SALES |
| NULL | OPERATIONS |
+--------+------------+
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？
正确。
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？
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 |
+--------+--------+
2.9、三张表，四张表怎么连接？
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。都可以出现！
案例：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？
select
e.ename,e.sal,d.dname,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 | sal | dname | grade |
+--------+---------+------------+-------+
| SMITH | 800.00 | RESEARCH | 1 |
| ALLEN | 1600.00 | SALES | 3 |
| WARD | 1250.00 | SALES | 2 |
| JONES | 2975.00 | RESEARCH | 4 |
| MARTIN | 1250.00 | SALES | 2 |
| BLAKE | 2850.00 | SALES | 4 |
| CLARK | 2450.00 | ACCOUNTING | 4 |
| SCOTT | 3000.00 | RESEARCH | 4 |
| KING | 5000.00 | ACCOUNTING | 5 |
| TURNER | 1500.00 | SALES | 3 |
| ADAMS | 1100.00 | RESEARCH | 1 |
| JAMES | 950.00 | SALES | 1 |
| FORD | 3000.00 | RESEARCH | 4 |
| MILLER | 1300.00 | ACCOUNTING | 2 |
+--------+---------+------------+-------+
案例：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？
select
e.ename,e.sal,d.dname,s.grade,l.ename
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 | sal | dname | grade | ename |
+--------+---------+------------+-------+-------+
| SMITH | 800.00 | RESEARCH | 1 | FORD |
| ALLEN | 1600.00 | SALES | 3 | BLAKE |
| WARD | 1250.00 | SALES | 2 | BLAKE |
| JONES | 2975.00 | RESEARCH | 4 | KING |
| MARTIN | 1250.00 | SALES | 2 | BLAKE |
| BLAKE | 2850.00 | SALES | 4 | KING |
| CLARK | 2450.00 | ACCOUNTING | 4 | KING |
| SCOTT | 3000.00 | RESEARCH | 4 | JONES |
| KING | 5000.00 | ACCOUNTING | 5 | NULL |
| TURNER | 1500.00 | SALES | 3 | BLAKE |
| ADAMS | 1100.00 | RESEARCH | 1 | SCOTT |
| JAMES | 950.00 | SALES | 1 | BLAKE |
| FORD | 3000.00 | RESEARCH | 4 | JONES |
| MILLER | 1300.00 | ACCOUNTING | 2 | CLARK |
+--------+---------+------------+-------+-------+
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