1.去重
示例表内容参考此文章
有些 mysql 数据表中可能存在重复的记录，有些情况我们允许重复数据的存在，但有时候我们也需要删除这些重复的数据。
例如：去重显示岗位信息：
mysql> select distinct job from emp;
+-----------+
| job |
+-----------+
| CLERK |
| SALESMAN |
| MANAGER |
| ANALYST |
| PRESIDENT |
+-----------+
5 rows in set (0.02 sec)
另一个示例：联合去重，查找部门和岗位的独有信息：
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 |
+-----------+--------+
9 rows in set (0.00 sec)
另一个示例：现在我们想统计一下工作岗位的数量，结合使用count函数：
mysql> select count(distinct job) from emp;
+---------------------+
| count(distinct job) |
+---------------------+
| 5 |
+---------------------+
1 row in set (0.00 sec)
2.连接查询
我们已经学会了如何在一张表中读取数据，这是相对简单的，但是在真正的应用中经常需要从多个数据表中读取数据。
JOIN 按照功能大致分为如下三类：
INNER JOIN（内连接,或等值连接）：获取两个表中字段匹配关系的记录。
LEFT JOIN（左连接）：获取左表所有记录，即使右表没有对应匹配的记录。
RIGHT JOIN（右连接）： 与 LEFT JOIN 相反，用于获取右表所有记录，即使左表没有对应匹配的记录。
多表连接的机制是：从其中一个表中取出每一条数据，从另一个表中的数据行进行匹配。这就涉及到了效率控制问题
使用where进行多表连接查询
现在我们来演示一个例子：取出每个员工的名字和部门名字：
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)上面的sql语句实际上效率很低，我们尝试进行优化（给表起别名）：（sql92语法）
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)注意：表的连接次数越多，效率越低，请尽量减少表的连接次数！
内连接 - 等值连接
还是上面的例子，取出每个员工的名字和部门名字：（sql99语法）
内连接，我们使用inner
mysql> select e.ename,d.dname
-> from emp e
-> inner 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)sql99的优点是：表的连接是独立的，不占用where的位置。使sql语句整体更加清晰
内连接 - 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
-> 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 (0.01 sec)内连接 - 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
我们可以发现，员工和领导的关系在一张表中，此时需要用到自连接（技巧：一张表看成两张表）
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)外连接 - 左右外连接
外连接与内连接的区别是，外连接没有匹配成功的某一个表的记录也会被取出
案例：查找员工的部门信息。要求部门即使没有员工也要查出
mysql> select
-> e.ename,d.dname
-> from emp e
-> right 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 |
| NULL | OPERATIONS |
+--------+------------+
15 rows in set (0.00 sec)同样的，如果是左外连接，将查询出左表的全部数据，使用left join关键字即可
外连接的查询结果条数一定是 >= 内连接的查询结果条数
三表连接
更为复杂的情况是，群表连接
我们来看一个案例：
找出每个员工的部门名称及工资等级。要求显示员工名，部门名，薪资，薪资等级
mysql> select
-> e.ename,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 |
+--------+---------+------------+-------+
14 rows in set (0.00 sec)再来看一个更复杂的情况：
找出每个员工的部门名称及工资等级及领导名称。要求显示员工名，部门名，领导名，薪资，薪资等级
mysql> 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 |
+--------+---------+------------+-------+-------+
14 rows in set (0.00 sec)以上就是MySQL数据库学习之去重与连接查询详解的详细内容，更多关于MySQL去重 连接查询的资料请关注我们其它相关文章！
