MySQL03-(进阶篇)
连接查询和子查询
主要讲 ：连接查询 ， 子查询 union limit
关于查询结果去重
没有去重之前:
mysql> select job from emp;
+-----------+
| job |
+-----------+
| CLERK |
| SALESMAN |
| SALESMAN |
| MANAGER |
| SALESMAN |
| MANAGER |
| MANAGER |
| ANALYST |
| PRESIDENT |
| SALESMAN |
| CLERK |
| CLERK |
| ANALYST |
| CLERK |
+-----------+
14 rows in set (0.00 sec)
在字段前面加上distinct关键字:
distinct只能出现在所有字段的最前端，distinct 字段 ,字段… 表示联合去重
mysql> select distinct job from emp;
+-----------+
| job |
+-----------+
| CLERK |
| SALESMAN |
| MANAGER |
| ANALYST |
| PRESIDENT |
+-----------+
5 rows in set (0.00 sec)
1.连接查询概述
1.什么是连接查询？
在实际开发中，大部分的情况下都不是从单表中查询数据，一般都是从多张表联合查询取出最终结果
2.连接查询的分类
- 内连接
- 等值连接
- 非等值连接
- 自连接
- 外连接
- 左外连接
- 右外连接
- 全连接（了解）
3.笛卡尔积现象
当两张表进行连接查询的时候，没有任何条件限制，最终的查询结果条数是两张表记录条数的乘积
案例：要求显示每个员工，和对应的部门名字
需要从这两张表中查询
emp表，共14条记录
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)
dept表，共4条记录
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)
当执行select e.ename,d.dname from emp e,dept d时候会出现56条记录，这就是笛卡儿积现象
mysql> 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 |
...
56 rows in set (0.00 sec)
4.避免迪卡尔现象，就是给查询加限定条件
mysql> select
-> e.ename,d.dname
-> from
-> emp e,dept d
-> where
-> e.deptno=d.deptno;
#注意：现在使用的是SQL92的语法，一般不用，只是用做演示
+--------+------------+
| 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 |
+--------+------------+
14 rows in set (0.00 sec)
避免了迪卡尔现象会减少记录的匹配次数吗？
不会，次数还是56次，只是显示的是有效记录
5.关于表的别名
select e.ename,d.dname from emp e,dept d;
好处：
执行效率高
可读性号
2.内连接
等值连接
还是上面的案例：要求显示每个员工，和对应的部门名字
这次我们使用SQL99的语法
select
e.ename,d.dname
from
emp e
join
dept d
on
e.deptno=d.deptno;
输出:
mysql> select e.ename,d.dname from emp e join dept d on e.deptno=d.deptno;
+--------+------------+
| 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 |
+--------+------------+
语法
..
A
join
B
on
连接条件
where
过滤条件
SQL99的语法更清晰：表的连接条件和后来的where条件分离了
非等值连接
案例:要求显示每个员工名，员工工资，员工工资等级
工资等级表salgrade
+-------+-------+-------+
| 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;
输出：
mysql> select e.ename,e.sal,s.grade from emp e 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 (0.00 sec)
自连接
最大的特点是：一张表当成两张表
案例：要求显示员工名字，和对应领导的名字
注意：领导也是员工，存在同一张表
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 |
+-------+--------+-----------+------+------------+---------+---------+--------+
select
e.ename as "员工",m.ename as "领导"
from
emp e
join
emp m
on
e.mgr=m.empno;
输出：
+--------+-------+
| 员工 | 领导 |
+--------+-------+
| 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)
里面没有KING,原因是KING没有上级领导
外连接
外连接与内连接有什么区别?
内连接:
假设A和B进行连接，使用内连接的话，凡是A表和B表能够匹配上的记录查询出来，这就是内连接。
外连接：
假设A和B表进行连接，使用外连接的话，AB两张表中有一张是主表一张是副表，主要查询主表中的数据，同时查询副标，当附表中的数据没有和主表中的数据匹配上，附表自动模拟出NULL与之匹配。
左连接：以左表为主表右连接：以右表为附表
解决上面案例的问题，查询结果没有KING,并不是我们想要的结果,
如果在join 加上left/right就表示左连接/右连接,以左/右表为主
select e.ename as "员工",m.ename as "领导" from emp e left join emp m on e.mgr=m.empno;
输出结果：
+--------+-------+
| 员工 | 领导 |
+--------+-------+
| 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 |
+--------+-------+
出现了KING，如果换成right,就是以领导表为主
select e.ename as "员工",m.ename as "领导" from emp e right join emp m on e.mgr=m.empno;
+--------+--------+
| 员工 | 领导 |
+--------+--------+
| NULL | SMITH |
| NULL | ALLEN |
| NULL | WARD |
| SCOTT | JONES |
| FORD | JONES |
| NULL | MARTIN |
| ALLEN | BLAKE |
| WARD | BLAKE |
| MARTIN | BLAKE |
| TURNER | BLAKE |
| JAMES | BLAKE |
| MILLER | CLARK |
| ADAMS | SCOTT |
| JONES | KING |
| BLAKE | KING |
| CLARK | KING |
| NULL | TURNER |
| NULL | ADAMS |
| NULL | JAMES |
| SMITH | FORD |
| NULL | MILLER |
+--------+--------+
案例：找出那个部门没有员工？
分析：肯定是以部门表为主表
先全部查出来
mysql> select e.* ,d.* from dept d left join emp e on d.deptno=e.deptno;
+-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
| EMPNO | ENAME | JOB | MGR | HIREDATE | SAL | COMM | DEPTNO | DEPTNO | DNAME | LOC |
+-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
| 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450.00 | NULL | 10 | 10 | ACCOUNTING | NEW YORK |
| 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 | 10 | ACCOUNTING | NEW YORK |
| 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300.00 | NULL | 10 | 10 | ACCOUNTING | NEW YORK |
| 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800.00 | NULL | 20 | 20 | RESEARCH | DALLAS |
| 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 | 20 | RESEARCH | DALLAS |
| 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL | 20 | 20 | RESEARCH | DALLAS |
| 7876 | ADAMS | CLERK | 7788 | 1987-05-23 | 1100.00 | NULL | 20 | 20 | RESEARCH | DALLAS |
| 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 | 20 | RESEARCH | DALLAS |
| 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600.00 | 300.00 | 30 | 30 | SALES | CHICAGO |
| 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250.00 | 500.00 | 30 | 30 | SALES | CHICAGO |
| 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250.00 | 1400.00 | 30 | 30 | SALES | CHICAGO |
| 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850.00 | NULL | 30 | 30 | SALES | CHICAGO |
| 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500.00 | 0.00 | 30 | 30 | SALES | CHICAGO |
| 7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950.00 | NULL | 30 | 30 | SALES | CHICAGO |
| NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | 40 | OPERATIONS | BOSTON |
+-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
15 rows in set (0.00 sec)
可以看到最后一个是我们想要的结果
用where筛选出来
mysql> select d.* from dept d left join emp e on d.deptno=e.deptno where e.deptno is null;
+--------+------------+--------+
| DEPTNO | DNAME | LOC |
+--------+------------+--------+
| 40 | OPERATIONS | BOSTON |
+--------+------------+--------+
1 row in set (0.00 sec)
三张表以及三张表以上的连接
语法结构
...
from
A
join
B
on
连接条件
join
c
on
连接条件
...
表示A先与B表连接，然后再和C表连接
案例：显示每个员工的名字，部门名称，工资等级
select
e.ename,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;
输出结果：
+--------+------------+-------+
| ename | dname | grade |
+--------+------------+-------+
| SMITH | RESEARCH | 1 |
| ALLEN | SALES | 3 |
| WARD | SALES | 2 |
| JONES | RESEARCH | 4 |
| MARTIN | SALES | 2 |
| BLAKE | SALES | 4 |
| CLARK | ACCOUNTING | 4 |
| SCOTT | RESEARCH | 4 |
| KING | ACCOUNTING | 5 |
| TURNER | SALES | 3 |
| ADAMS | RESEARCH | 1 |
| JAMES | SALES | 1 |
| FORD | RESEARCH | 4 |
| MILLER | ACCOUNTING | 2 |
+--------+------------+-------+
14 rows in set (0.00 sec)
案例:显示每个员工的名字，部门名称，工资等级,以及领导名字：
分析：加入外连接，以emp表为主表
select
e.ename,d.dname,s.grade,m.ename as "manager"
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 m
on
e.mgr=m.empno;
+--------+------------+-------+---------+
| ename | dname | grade | manager |
+--------+------------+-------+---------+
| SMITH | RESEARCH | 1 | FORD |
| ALLEN | SALES | 3 | BLAKE |
| WARD | SALES | 2 | BLAKE |
| JONES | RESEARCH | 4 | KING |
| MARTIN | SALES | 2 | BLAKE |
| BLAKE | SALES | 4 | KING |
| CLARK | ACCOUNTING | 4 | KING |
| SCOTT | RESEARCH | 4 | JONES |
| KING | ACCOUNTING | 5 | NULL |
| TURNER | SALES | 3 | BLAKE |
| ADAMS | RESEARCH | 1 | SCOTT |
| JAMES | SALES | 1 | BLAKE |
| FORD | RESEARCH | 4 | JONES |
| MILLER | ACCOUNTING | 2 | CLARK |
+--------+------------+-------+---------+
14 rows in set (0.00 sec)
子查询
就是在sql语句中，再嵌套select,被嵌套的select语句就是子查询
在where 中嵌套select
案例:显示出多大于平均工资的员工姓名
select ename from emp where sal>(select avg(sal) from emp);
输出结果
+-------+
| ename |
+-------+
| JONES |
| BLAKE |
| CLARK |
| SCOTT |
| KING |
| FORD |
+-------+
6 rows in set (0.01 sec)
在from 中嵌套select
显示出每个部门的平均工资，以及平均工资的工资等级
分析：应该分两部来做
1.显示每个部门的平均工资
select deptno, avg(sal) as 'avg' from emp group by deptno;
+--------+-------------+
| deptno | avg |
+--------+-------------+
| 10 | 2916.666667 |
| 20 | 2175.000000 |
| 30 | 1566.666667 |
+--------+-------------+
3 rows in set (0.00 sec)
2.联合salgrade表
mysql> select * from salgrade;
+-------+-------+-------+
| GRADE | LOSAL | HISAL |
+-------+-------+-------+
| 1 | 700 | 1200 |
| 2 | 1201 | 1400 |
| 3 | 1401 | 2000 |
| 4 | 2001 | 3000 |
| 5 | 3001 | 9999 |
+-------+-------+-------+
最终的写法是
select
a.*,s.grade
from
(select deptno, avg(sal) as 'avg' from emp group by deptno) a
left join
salgrade s
on
a.avg between s.losal and s.hisal;
+--------+-------------+-------+
| deptno | avg | grade |
+--------+-------------+-------+
| 10 | 2916.666667 | 4 |
| 20 | 2175.000000 | 4 |
| 30 | 1566.666667 | 3 |
+--------+-------------+-------+
3 rows in set (0.00 sec)
3.在select中嵌套select
union
可以将结果集合并
案例:找出工作岗位为salesman 和 manager的员工
有三种方法：
select ename from emp where job='salesman' or job='manager';
select ename from emp where job in ('salesman','manager')
3.使用合并
select ename from emp where job='salesman'
union
select ename from emp where job='manager';
输出结果都是
+--------+
| ename |
+--------+
| ALLEN |
| WARD |
| MARTIN |
| TURNER |
| JONES |
| BLAKE |
| CLARK |
+--------+
7 rows in set (0.01 sec)
union还可以合并两张不相干的表
比如将部门名与员工名合并
mysql> select ename from emp
-> union
-> select dname from dept;
+------------+
| ename |
+------------+
| SMITH |
| ALLEN |
| WARD |
| JONES |
| MARTIN |
| BLAKE |
| CLARK |
| SCOTT |
| KING |
| TURNER |
| ADAMS |
| JAMES |
| FORD |
| MILLER |
| ACCOUNTING |
| RESEARCH |
| SALES |
| OPERATIONS |
+------------+
18 rows in set (0.00 sec)
*limit
取结果集中的部分数据，相当于切片，
limit是mysql中特有的，其他数据库中没有
语法机制
limit startIndex,length
startIndex表示起始的位置
length表示取几个
案例取出工资前5名的员工
select ename,sal from emp order by sal desc limit 0 ,5;
+-------+---------+
| ename | sal |
+-------+---------+
| KING | 5000.00 |
| SCOTT | 3000.00 |
| FORD | 3000.00 |
| JONES | 2975.00 |
| BLAKE | 2850.00 |
+-------+---------+
案例取出排名在5到10的员工
select ename,sal from emp order by sal desc limit 5 ,5;
+--------+---------+
| ename | sal |
+--------+---------+
| CLARK | 2450.00 |
| ALLEN | 1600.00 |
| TURNER | 1500.00 |
| MILLER | 1300.00 |
| MARTIN | 1250.00 |
+--------+---------+
应用:在实际的项目开发中，经常用到limit做分页数据显示
一个公式: 页码：pageno 一页显示的记录pagesize
limit应该这么写：
limit (pageno-1)*pagesize , pagesize