MySQL数据库学习之去重与连接查询详解
作者:世界尽头与你
1.去重
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)
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