查询结果集的去重
/*
mysql> select distinct job from emp;// distinct关键字去重重复记录。
+-----------+
| job |
+-----------+
| CLERK |
| SALESMAN |
| MANAGER |
| ANALYST |
| PRESIDENT |
+-----------+
mysql> select ename, distinct job from emp;
以上的sql语句是错误的。
记住：distinct只能出现在所有字段的最前面。
mysql> select distinct deptno,job from emp;
deptno和job联合去重
+--------+-----------+
| deptno | job |
+--------+-----------+
| 20 | CLERK |
| 30 | SALESMAN |
| 20 | MANAGER |
| 30 | MANAGER |
| 10 | MANAGER |
| 20 | ANALYST |
| 10 | PRESIDENT |
| 30 | CLERK |
| 10 | CLERK |
+--------+-----------+
案例：统计岗位的数量？
select count(distinct job) from emp;
+---------------------+
| count(distinct job) |
+---------------------+
| 5 |
+---------------------+
*/连接查询
/*
2.1、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 高三1班
2 ls 1 高三2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。
2.2、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！）
2.3、在表的连接查询方面有一种现象称为：笛卡尔积现象。（笛卡尔乘积现象）
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 |
...
56 rows in set (0.00 sec)
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。
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。
2.4、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; //SQL92，以后不用
+--------+------------+
| 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 |
+--------+------------+
2.5、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
join
dept d
on
e.deptno = d.deptno;
// inner可以省略，带着inner的目的是可读性更强一些。
select
e.ename, d.dname
from
emp e
inner join
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。
+--------+------------+
| 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 |
+--------+------------+
2.6、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。
// 员工表
mysql> select ename,sal from emp;
+--------+---------+
| 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 |
+--------+---------+
// 工资等级表
mysql> select * from 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;
// inner可以省略
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：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 | // mar --> FORD
| 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 a
inner 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条记录，KING没有上级领导就没显示
2.8、外连接
什么是外连接，和内连接有什么区别？
内连接：
假设A和B表进行内连接，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。
AB两张表没有主副之分，两张表是平等的。
外连接：
假设A和B表进行连接，使用外连接的话，AB两张表中有一张表是主表，一张表是副表，主要查询主表中
的数据。捎带着查询副表，当副表中的数据没有和主表中的数据匹配上，副表自动模拟出NULL与之匹配。
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。
左连接有右连接的写法，右连接也会有对应的左连接的写法。
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 a
inner join
emp b
on
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 a
left join
emp b
on
a.mgr = b.empno;
// outer可以省略
select
a.ename '员工', b.ename '领导'
from
emp a
left outer join
emp b
on
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 b
right join
emp a
on
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 |
+--------+-------+
// 14条记录，KING仍然显示出来，副表为NULL
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。
案例：找出哪个部门没有员工？
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)
mysql> select * from dept;
+--------+------------+----------+
| DEPTNO | DNAME | LOC |
+--------+------------+----------+
| 10 | ACCOUNTING | NEW YORK |
| 20 | RESEARCH | DALLAS |
| 30 | SALES | CHICAGO |
| 40 | OPERATIONS | BOSTON |
+--------+------------+----------+
select
e.*, d.*
from
emp e
right join
dept d
on
e.deptno = d.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 |
+-------+--------+-----------+------+------------+---------+---------+--------+--------+------------+----------+
继续加where查询条件：
select
e.*, d.*
from
emp e
right join
dept d
on
e.deptno = d.deptno
where
e.empno is null;
+-------+-------+------+------+----------+------+------+--------+--------+------------+--------+
| EMPNO | ENAME | JOB | MGR | HIREDATE | SAL | COMM | DEPTNO | DEPTNO | DNAME | LOC |
+-------+-------+------+------+----------+------+------+--------+--------+------------+--------+
| NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | 40 | OPERATIONS | BOSTON |
+-------+-------+------+------+----------+------+------+--------+--------+------------+--------+
题意：保留d.*即可。
select
d.*
from
emp e
right join
dept d
on
e.deptno = d.deptno
where
e.empno is null;
+--------+------------+--------+
| DEPTNO | DNAME | LOC |
+--------+------------+--------+
| 40 | OPERATIONS | BOSTON |
+--------+------------+--------+
2.9、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。
mysql> select empno,ename,deptno,sal from emp;
EMP e
+-------+--------+--------+---------+
| empno | ename | deptno | sal |
+-------+--------+--------+---------+
| 7369 | SMITH | 20 | 800.00 |
| 7499 | ALLEN | 30 | 1600.00 |
| 7521 | WARD | 30 | 1250.00 |
| 7566 | JONES | 20 | 2975.00 |
| 7654 | MARTIN | 30 | 1250.00 |
| 7698 | BLAKE | 30 | 2850.00 |
| 7782 | CLARK | 10 | 2450.00 |
| 7788 | SCOTT | 20 | 3000.00 |
| 7839 | KING | 10 | 5000.00 |
| 7844 | TURNER | 30 | 1500.00 |
| 7876 | ADAMS | 20 | 1100.00 |
| 7900 | JAMES | 30 | 950.00 |
| 7902 | FORD | 20 | 3000.00 |
| 7934 | MILLER | 10 | 1300.00 |
+-------+--------+--------+---------+
mysql> select * from dept;
DEPT d
+--------+------------+----------+
| DEPTNO | DNAME | LOC |
+--------+------------+----------+
| 10 | ACCOUNTING | NEW YORK |
| 20 | RESEARCH | DALLAS |
| 30 | SALES | CHICAGO |
| 40 | OPERATIONS | BOSTON |
+--------+------------+----------+
mysql> select * from salgrade;
SALGRADE s
+-------+-------+-------+
| GRADE | LOSAL | HISAL |
+-------+-------+-------+
| 1 | 700 | 1200 |
| 2 | 1201 | 1400 |
| 3 | 1401 | 2000 |
| 4 | 2001 | 3000 |
| 5 | 3001 | 9999 |
+-------+-------+-------+
注意，解释一下：
...
A
join
B
join
C
on
...
表示：A表和B表先进行表连接，连接之后A表继续和C表进行连接。
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;
+--------+------------+-------+
| 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 |
+--------+------------+-------+
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 '部门名称', s.grade '工资等级', 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;
+--------+------------+----------+-------+
| 员工 | 部门名称 | 工资等级 | 领导 |
+--------+------------+----------+-------+
| 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 |
+--------+------------+----------+-------+
3、子查询
3.1、什么是子查询？子查询都可以出现在哪里？
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)
3.2、where字句中使用了子查询
案例：找出高于平均薪资的员工信息。
select * from emp sal > avg(sal);// 错误的写法，where后面不能直接使用分组函数。
select * from emp where sal > (select avg(sal) from emp);
3.3、from后面嵌套子查询
案例：找出每个部门平均薪水的薪资等级。
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.hisal。
select
t.*, s.grade
from
(select deptno,avg(sal) as avgsal from emp group by deptno) t
join
salgrade s
on
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。
mysql>
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.hisal
group by
e.deptno;
+--------+--------------+
| deptno | avg(s.grade) |
+--------+--------------+
| 10 | 3.6667 |
| 20 | 2.8000 |
| 30 | 2.5000 |
+--------+--------------+
3.4、在select后面嵌套子查询。
实例：找出每个员工所在的部门名称，要求显示员工名和部门名。
不用连接表的方式，使用嵌套查询方式。
select
e.ename, e.deptno, (select d.dname from dept d where e.deptno = d.deptno) as dname
from
emp e;
+--------+--------+------------+
| ename | deptno | dname |
+--------+--------+------------+
| SMITH | 20 | RESEARCH |
| ALLEN | 30 | SALES |
| WARD | 30 | SALES |
| JONES | 20 | RESEARCH |
| MARTIN | 30 | SALES |
| BLAKE | 30 | SALES |
| CLARK | 10 | ACCOUNTING |
| SCOTT | 20 | RESEARCH |
| KING | 10 | ACCOUNTING |
| TURNER | 30 | SALES |
| ADAMS | 20 | RESEARCH |
| JAMES | 30 | SALES |
| FORD | 20 | RESEARCH |
| MILLER | 10 | ACCOUNTING |
+--------+--------+------------+
4、union（可以将查询结果相加）
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 = "salesman" or job = "manager";
第二种：select ename,job from emp where job in ("salesman", "manager");
+--------+----------+
| ename | job |
+--------+----------+
| ALLEN | SALESMAN |
| WARD | SALESMAN |
| JONES | MANAGER |
| MARTIN | SALESMAN |
| BLAKE | MANAGER |
| CLARK | MANAGER |
| TURNER | SALESMAN |
+--------+----------+
第三种：union
select ename,job from emp where job = "salesman"
union
select ename,job from emp where job = "manager";
两张不相干的表中的数据拼接在一起显示？
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 |
+------------+
注意：union相加时，两方列数相同
mysql> select ename,sal from emp
-> union
-> select dname from dept;
ERROR 1222 (21000): The used SELECT statements have a different number of columns
5、limit（重点中的重点：以后分页查询全靠它了）
5.1、limit是mysql特有的，其他数据库中没有，不通用。
5.2、limit取结果集中的部分数据，这是它的作用。
5.3、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;
+-------+---------+
| ename | sal |
+-------+---------+
| KING | 5000.00 |
| SCOTT | 3000.00 |
| FORD | 3000.00 |
| JONES | 2975.00 |
| BLAKE | 2850.00 |
+-------+---------+
5.4、limit是sql语句最后执行的一个环节：
select 5
...
from 1
...
where 2
...
group by 3
...
having 4
...
order by 6
...
limit 7
...;
5.5、案例：找到工资排名在第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 |
+--------+---------+
5.6、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
第4页：9， 3
第5页：12， 3
...
每页显示pageSize条记录：
第pageNo页：(pageNo-1) * pageSize, pageSize
pageSize是什么：每页显示多少条记录
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;
}
*/
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