【MySQL】笔记（3）--- 连接查询；子查询；union；limit；-百木园

一.连接查询：

1.1、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，一般都是多张表联合查询取出最终的结果。在实际开发中，一般一个业务都会对应多张表，比如：学生和班级，起码两张表。 stuno stuname classno classname ----------------------------------------------------------------------------------- 1 zs 1 北京大兴区亦庄经济技术开发区第二中学高三1班 2 ls 1 北京大兴区亦庄经济技术开发区第二中学高三1班 ... 学生和班级信息存储到一张表中，结果就像上面一样，数据会存在大量的重复，导致数据的冗余。

1.2、连接查询的分类？根据语法出现的年代来划分的话，包括： SQL92（一些老的DBA可能还在使用这种语法,DBA：DataBase Administrator，数据库管理员） SQL99（比较新的语法）根据表的连接方式来划分，包括：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（这个不讲，很少用！）

1.3、在表的连接查询方面有一种现象被称为：笛卡尔积现象 / 笛卡尔乘积现象

笛卡尔积现象：当两张表进行连接查询的时候，没有任何条件进行限制，最终的查询结果条数是两张表记录条数的乘积。

案例：找出每一个员工的部门名称，要求显示员工名和部门名。EMP表+--------+--------+| 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 |+--------+--------+DEPT表+--------+------------+----------+| DEPTNO | DNAME | LOC |+--------+------------+----------+| 10 | ACCOUNTING | NEW YORK || 20 | RESEARCH | DALLAS || 30 | SALES | CHICAGO || 40 | OPERATIONS | BOSTON |+--------+------------+----------+

select ename,dname from emp,dept;+--------+------------+| ename | dname |+--------+------------+| SMITH | ACCOUNTING || SMITH | RESEARCH || SMITH | SALES || SMITH | OPERATIONS || ALLEN | ACCOUNTING || ALLEN | RESEARCH || ALLEN | SALES || ALLEN | OPERATIONS |............( 4X14 )

关于表的别名： select e.ename,d.dname from emp e,dept d; 表的别名有什么好处？第一：执行效率高。第二：可读性好。

1.4、怎么避免笛卡尔积现象？当然是加条件进行过滤。思考：避免了笛卡尔积现象，会减少记录的匹配次数吗？不会，次数还是56次。只不过显示的是有效记录。

案例：找出每一个员工的部门名称，要求显示员工名和部门名。 select e.ename,d.dname from emp e,dept d where e.deptno = d.deptno;

1.5、内连接之等值连接：最大特点是：条件是等量关系。

案例：查询每个员工的部门名称，要求显示员工名和部门名。

SQL92:（太老，不用了） select e.ename,d.dname from emp e, dept d where e.deptno = d.deptno;

SQL99：（常用的） select e.ename,d.dname from emp e inner join // inner可以省略的，带着inner目的是可读性好一些 dept d on e.deptno = d.deptno;

语法： ... A join B on 连接条件 where ... SQL99语法结构更清晰一些：表的连接条件和后来的where条件分离了。

1.6、内连接之非等值连接：最大的特点是：连接条件中的关系是非等量关系。

案例：找出每个员工的工资等级，要求显示员工名、工资、工资等级。mysql> select ename,sal from emp e+--------+---------+| ename | sal |+--------+---------+| SMITH | 800.00 || ALLEN | 1600.00 || WARD | 1250.00 || JONES | 2975.00 || MARTIN | 1250.00 || BLAKE | 2850.00 || CLARK | 2450.00 || SCOTT | 3000.00 || KING | 5000.00 || TURNER | 1500.00 || ADAMS | 1100.00 || JAMES | 950.00 || FORD | 3000.00 || MILLER | 1300.00 |+--------+---------+

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.gradefrom emp e(inner) join salgrade son 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 |+--------+---------+-------+

1.7、自连接：最大的特点是：一张表看做两张表。自己连接自己。

案例：找出每个员工的上级领导，要求显示员工名和对应的领导名。mysql> select empno,ename,mgr from emp;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 |+-------+--------+| 7566 | JONES || 7698 | BLAKE || 7782 | CLARK || 7788 | SCOTT || 7839 | KING || 7902 | FORD |+-------+--------+

员工的领导编号 = 领导的员工编号

select a.ename as \'员工名\',b.ename as \'领导名\'from emp ainner join emp bon 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 |+--------+--------+

1.8、外连接: 外连接的分类？左外连接（左连接）：表示左边的这张表是主表。右外连接（右连接）：表示右边的这张表是主表。

案例：找出每个员工的上级领导？（所有员工必须全部查询出来）

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 |+-------+--------+| 7566 | JONES || 7698 | BLAKE || 7782 | CLARK || 7788 | SCOTT || 7839 | KING || 7902 | FORD |+-------+--------+

内连接：select a.ename \'员工\', b.ename \'领导\'from emp ajoin emp bon a.mgr = b.empno;

外连接：（左外连接/左连接）select a.ename \'员工\', b.ename \'领导\'from emp aleft （outer）join emp bon a.mgr = b.empno;

外连接：（右外连接/右连接）select a.ename \'员工\', b.ename \'领导\'from emp bright （outer）join emp aon a.mgr = b.empno;

+--------+-------+| 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 |+--------+-------+

外连接最重要的特点是：主表的数据无条件的全部查询出来。

1.9、三张表怎么连接查询？案例：找出每一个员工的部门名称以及工资等级。EMP e+-------+--------+---------+--------+| empno | ename | sal | deptno |+-------+--------+---------+--------+| 7369 | SMITH | 800.00 | 20 || 7499 | ALLEN | 1600.00 | 30 || 7521 | WARD | 1250.00 | 30 || 7566 | JONES | 2975.00 | 20 || 7654 | MARTIN | 1250.00 | 30 || 7698 | BLAKE | 2850.00 | 30 || 7782 | CLARK | 2450.00 | 10 || 7788 | SCOTT | 3000.00 | 20 || 7839 | KING | 5000.00 | 10 || 7844 | TURNER | 1500.00 | 30 || 7876 | ADAMS | 1100.00 | 20 || 7900 | JAMES | 950.00 | 30 || 7902 | FORD | 3000.00 | 20 || 7934 | MILLER | 1300.00 | 10 |+-------+--------+---------+--------+DEPT d+--------+------------+----------+| DEPTNO | DNAME | LOC |+--------+------------+----------+| 10 | ACCOUNTING | NEW YORK || 20 | RESEARCH | DALLAS || 30 | SALES | CHICAGO || 40 | OPERATIONS | BOSTON |+--------+------------+----------+SALGRADE s+-------+-------+-------+| GRADE | LOSAL | HISAL |+-------+-------+-------+| 1 | 700 | 1200 || 2 | 1201 | 1400 || 3 | 1401 | 2000 || 4 | 2001 | 3000 || 5 | 3001 | 9999 |+-------+-------+-------+

案例：找出每一个员工的部门名称、工资等级、以及上级领导。 select e.ename \'员工\',d.dname,s.grade,e1.ename \'领导\' 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 e1 on e.mgr = e1.empno;

二.子查询：

2.1、select语句当中嵌套select语句，被嵌套的select语句是子查询。子查询可以出现在哪里？ select ..(select). from ..(select). where ..(select).

2.2、where子句中使用子查询

案例：找出高于平均薪资的员工信息。select * from emp where sal > avg(sal); //错误的写法，where后面不能直接使用分组函数

第一步：找出平均薪资 select avg(sal) from emp; +-------------+ | avg(sal) | +-------------+ | 2073.214286 | +-------------+第二步：where过滤 select * from emp where sal > 2073.214286; +-------+-------+-----------+------+------------+---------+------+--------+ | EMPNO | ENAME | JOB | MGR | HIREDATE | SAL | COMM | DEPTNO | +-------+-------+-----------+------+------------+---------+------+--------+ | 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 | | 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 | | 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 | +-------+-------+-----------+------+------------+---------+------+--------+第一步和第二步合并： select * from emp where sal > (select avg(sal) from emp);

2.3、from后面嵌套子查询

案例：找出每个部门平均薪水的等级。第一步：找出每个部门平均薪水（按照部门编号分组，求sal的平均值）select deptno,avg(sal) as avgsal from emp group by deptno;+--------+-------------+| deptno | avgsal |+--------+-------------+| 10 | 2916.666667 || 20 | 2175.000000 || 30 | 1566.666667 |+--------+-------------+第二步：将以上的查询结果当做临时表t，让t表和salgrade s表连接，条件是：t.avgsal between s.losal and s.hisalselect t.*,s.gradefrom (select deptno,avg(sal) as avgsal from emp group by deptno) tjoin salgrade son t.avgsal between s.losal and s.hisal;

+--------+-------------+-------+| deptno | avgsal | grade |+--------+-------------+-------+| 30 | 1566.666667 | 3 || 10 | 2916.666667 | 4 || 20 | 2175.000000 | 4 |+--------+-------------+-------+

案例：找出每个部门平均的薪水等级。第一步：找出每个员工的薪水等级。select e.ename,e.sal,e.deptno,s.grade from emp e join salgrade s on e.sal between s.losal and s.hisal;+--------+---------+--------+-------+| ename | sal | deptno | grade |+--------+---------+--------+-------+| SMITH | 800.00 | 20 | 1 || ALLEN | 1600.00 | 30 | 3 || WARD | 1250.00 | 30 | 2 || JONES | 2975.00 | 20 | 4 || MARTIN | 1250.00 | 30 | 2 || BLAKE | 2850.00 | 30 | 4 || CLARK | 2450.00 | 10 | 4 || SCOTT | 3000.00 | 20 | 4 || KING | 5000.00 | 10 | 5 || TURNER | 1500.00 | 30 | 3 || ADAMS | 1100.00 | 20 | 1 || JAMES | 950.00 | 30 | 1 || FORD | 3000.00 | 20 | 4 || MILLER | 1300.00 | 10 | 2 |+--------+---------+--------+-------+第二步：基于以上结果，继续按照deptno分组，求grade平均值。select e.deptno,avg(s.grade)from emp e join salgrade s on e.sal between s.losal and s.hisalgroup by e.deptno;

+--------+--------------+| deptno | avg(s.grade) |+--------+--------------+| 10 | 3.6667 || 20 | 2.8000 || 30 | 2.5000 |+--------+--------------+

2.4、在select后面嵌套子查询。案例：找出每个员工所在的部门名称，要求显示员工名和部门名。

//方法一：

select e.ename,d.dname

from emp e join dept d on e.deptno = d.deptno;

//方法二：（select后嵌套子查询）

select e.ename,(select d.dname from dept d where e.deptno = d.deptno) as dname from emp e;

+--------+------------+| 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 |+--------+------------+

三.union：

3.1union:拼接查询结果；

案例：找出工作岗位是SALESMAN和MANAGER的员工？第一种：select ename,job from emp where job = \'MANAGER\' or job = \'SALESMAN\';第二种：select ename,job from emp where job in(\'MANAGER\',\'SALESMAN\');+--------+----------+| ename | job |+--------+----------+| ALLEN | SALESMAN || WARD | SALESMAN || JONES | MANAGER || MARTIN | SALESMAN || BLAKE | MANAGER || CLARK | MANAGER || TURNER | SALESMAN |+--------+----------+第三种：unionselect ename,job from emp where job = \'MANAGER\'unionselect ename,job from emp where job = \'SALESMAN\';+--------+----------+| ename | job |+--------+----------+| JONES | MANAGER || BLAKE | MANAGER || CLARK | MANAGER || ALLEN | SALESMAN || WARD | SALESMAN || MARTIN | SALESMAN || TURNER | SALESMAN |+--------+----------+

3.2两张不相干的表中的数据拼接在一起显示？select ename from empunionselect dname from dept;

//毫无意义：

+------------+| ename |+------------+| SMITH || ALLEN || WARD || JONES || MARTIN || BLAKE || CLARK || SCOTT || KING || TURNER || ADAMS || JAMES || FORD || MILLER || ACCOUNTING || RESEARCH || SALES || OPERATIONS |+------------+

3.3 mysql> select ename,sal from emp -> union -> select dname from dept;ERROR 1222 (21000): The used SELECT statements have a different number of columns（列数不对应）

四.limit ：

4.1、limit是mysql特有的，其他数据库中没有，不通用（Oracle中有一个相同的机制，叫做rownum）；

4.2、limit取结果集中的部分数据，这是它的作用（分页）；

4.3、语法机制： limit startIndex, length startIndex表示起始位置，从0开始，0表示第一条数据。 length表示取几个案例：取出工资前5名的员工（思路：降序取前5个） select ename,sal from emp order by sal desc; 取前5个： select ename,sal from emp order by sal desc limit 0, 5; select ename,sal from emp order by sal desc limit 5;

//limit是sql语句最后执行的一个环节;

4.4、案例：找出工资排名在第4到第9名的员工？ select ename,sal from emp order by sal desc limit 3,6; +--------+---------+ | ename | sal | +--------+---------+ | JONES | 2975.00 | | BLAKE | 2850.00 | | CLARK | 2450.00 | | ALLEN | 1600.00 | | TURNER | 1500.00 | | MILLER | 1300.00 | +--------+---------+

4.5、通用的标准分页sql？

每页显示3条记录：第1页：0, 3第2页：3, 3第3页：6, 3 ......

每页显示pageSize条记录： //pageSize是什么？是每页显示多少条记录第pageNo页：(pageNo - 1) * pageSize, pageSize //pageNo是什么？显示第几页

java代码{ int pageNo = 2; // 页码是2 int pageSize = 10; // 每页显示10条 limit (pageNo - 1) * pageSize, pageSize}

资料出处：https://www.bilibili.com/video/BV1fx411X7BD

来源：https://www.cnblogs.com/Burning-youth/p/15677875.html
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【MySQL】笔记（3）--- 连接查询；子查询；union；limit；

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