MySQL复合查询和内外连接

文章目录

  • 一、复合查询
    • 1.1 多表查询
    • 1.2 自连接
    • 1.3 子查询
      • 1.3.1 单行子查询
      • 1.3.2 多行子查询
      • 1.3.3 多列子查询
      • 1.3.4 在from子句中使用子查询
    • 1.4 合并查询
      • 1.4.1 union
      • 1.4.2 union all
  • 二、内外连接
    • 2.1 内连接
    • 2.2 外连接
      • 2.2.1 左外连接
      • 2.2.1 右外连接


一、复合查询

1.1 多表查询

实际开发中往往数据来自不同的表,所以需要多表查询,但是可以将多张表做笛卡尔积后的表当做是一张表,也就是单表查询
我们用一个简单的公司管理系统,有三张表EMP、DEPT、SALGRADE来演示如何进行多表查询。

语法:from 表1, 表2 ...
这样其实就是对多张表进行笛卡尔积,假设是两张表:

  • 第一张表的第一行和第二张表的所有行组合。
  • 第一张表的第二行和第二张表的所有行组合。

MySQL复合查询和内外连接_第1张图片
这样所有的组合,并不都是有意义的,所有一般都是根据条件筛选的。

  • 在多表查询中,有可能多张表有相同的字段,为了便于区分,可以使用表名.字段名表示某一张表的某一个字段。
mysql> select dept.dname, emp.ename, emp.sal from emp,dept where dept.deptno = emp.deptno;
+------------+--------+---------+
| dname      | ename  | sal     |
+------------+--------+---------+
| RESEARCH   | SMITH  |  800.00 |
| SALES      | ALLEN  | 1600.00 |
| SALES      | WARD   | 1250.00 |
| RESEARCH   | JONES  | 2975.00 |
| SALES      | MARTIN | 1250.00 |
| SALES      | BLAKE  | 2850.00 |
| ACCOUNTING | CLARK  | 2450.00 |
| RESEARCH   | SCOTT  | 3000.00 |
| ACCOUNTING | KING   | 5000.00 |
| SALES      | TURNER | 1500.00 |
| RESEARCH   | ADAMS  | 1100.00 |
| SALES      | JAMES  |  950.00 |
| RESEARCH   | FORD   | 3000.00 |
| ACCOUNTING | MILLER | 1300.00 |
+------------+--------+---------+
14 rows in set (0.00 sec)

1.2 自连接

自连接是对一张表的查询,但是使用多张表的查询方式,自己做笛卡尔积。
例如:

mysql> show create table emp \G
*************************** 1. row ***************************
       Table: emp
Create Table: CREATE TABLE `emp` (
  `empno` int(6) unsigned zerofill NOT NULL COMMENT '雇员编号',
  `ename` varchar(10) DEFAULT NULL COMMENT '雇员姓名',
  `job` varchar(9) DEFAULT NULL COMMENT '雇员职位',
  `mgr` int(4) unsigned zerofill DEFAULT NULL COMMENT '雇员领导编号',
  `hiredate` datetime DEFAULT NULL COMMENT '雇佣时间',
  `sal` decimal(7,2) DEFAULT NULL COMMENT '工资月薪',
  `comm` decimal(7,2) DEFAULT NULL COMMENT '奖金',
  `deptno` int(2) unsigned zerofill DEFAULT NULL COMMENT '部门编号'
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

  • 显示员工FORD的上级领导的编号和姓名(mgr是员工领导的编号)。
    这是员工表,领导也是属于员工,所以无法只用一条简单查找语句对一张表查找两次,这就需要用到复合查询。

  • 子查询 — 即嵌套查询,使用两条查询语句

mysql> select empno, ename from emp where empno = (select mgr from emp where ename = 'FORD');
+--------+-------+
| empno  | ename |
+--------+-------+
| 007566 | JONES |
+--------+-------+
1 row in set (0.00 sec
  • 使用自连接的两张表查询,这里需要对表使用别名
mysql> select leader.empno, leader.ename from emp as worker, emp leader where worker.mgr = leader.empno and worker.ename='FORD';
+--------+-------+
| empno  | ename |
+--------+-------+
| 007566 | JONES |
+--------+-------+
1 row in set (0.00 sec)

1.3 子查询

子查询是指嵌入在其他sql语句中的select语句,也叫嵌套查询。

1.3.1 单行子查询

返回一行记录的子查询

  • 显示SMITH同一部门的员工
mysql> select deptno, ename from emp where deptno = (select deptno from emp where ename = 'SMITH');
+--------+-------+
| deptno | ename |
+--------+-------+
|     20 | SMITH |
|     20 | JONES |
|     20 | SCOTT |
|     20 | ADAMS |
|     20 | FORD  |
+--------+-------+
5 rows in set (0.00 sec)

1.3.2 多行子查询

返回多行记录的子查询,仍然是一列(一个字段)。

  • in关键字(属于查询出来的多行中的一行,无法用于比较):查询和10号部门的工作岗位相同的雇员的名字,岗位,工资,部门号,但是不包含10自己的
    先拆分,找到10号部门的岗位:
mysql> select distinct job from emp where deptno = 10;
+-----------+
| job       |
+-----------+
| MANAGER   |
| PRESIDENT |
| CLERK     |
+-----------+
3 rows in set (0.00 sec)
-- 然后再复合
mysql> select ename, job, sal, deptno from emp where job in (select distinct job from emp where deptno = 10) and deptno <=> 10;
+--------+-----------+---------+--------+
| ename  | job       | sal     | deptno |
+--------+-----------+---------+--------+
| CLARK  | MANAGER   | 2450.00 |     10 |
| KING   | PRESIDENT | 5000.00 |     10 |
| MILLER | CLERK     | 1300.00 |     10 |
+--------+-----------+---------+--------+
3 rows in set (0.00 sec)
  • all关键字(与查询结果的所有行比较为真的):显示工资比部门30的所有员工的工资高的员工的姓名、工资和部门号
    同样可以拆分
-- 先找到部门号为30的所有工资
mysql> select sal from emp where deptno = 30;
+---------+
| sal     |
+---------+
| 1600.00 |
| 1250.00 |
| 1250.00 |
| 2850.00 |
| 1500.00 |
|  950.00 |
+---------+
6 rows in set (0.00 sec)
-- 判断 > 所有行
mysql> select ename, sal, deptno from emp where sal > all(select sal from emp where deptno = 30);
+-------+---------+--------+
| ename | sal     | deptno |
+-------+---------+--------+
| JONES | 2975.00 |     20 |
| SCOTT | 3000.00 |     20 |
| KING  | 5000.00 |     10 |
| FORD  | 3000.00 |     20 |
+-------+---------+--------+
4 rows in set (0.00 sec)
  • any关键字(与查询结果的任意一行比较为真的):显示工资比部门30的任意员工的工资高的员工的姓名、工资和部门号(包含自己部门的员工) some是any的别称很少使用。
mysql> select ename, sal, deptno from emp where sal > any (select sal from emp where deptno=30);
+--------+---------+--------+
| ename  | sal     | deptno |
+--------+---------+--------+
| ALLEN  | 1600.00 |     30 |
| WARD   | 1250.00 |     30 |
| JONES  | 2975.00 |     20 |
| MARTIN | 1250.00 |     30 |
| BLAKE  | 2850.00 |     30 |
| CLARK  | 2450.00 |     10 |
| SCOTT  | 3000.00 |     20 |
| KING   | 5000.00 |     10 |
| TURNER | 1500.00 |     30 |
| ADAMS  | 1100.00 |     20 |
| FORD   | 3000.00 |     20 |
| MILLER | 1300.00 |     10 |
+--------+---------+--------+
12 rows in set (0.00 sec)

1.3.3 多列子查询

单行子查询是指子查询只返回单列,单行数据
多行子查询是指返回单列多行数据,都是针对单列而言的,而多列子查询则是指查询返回多个列数据的子查询语句

其实很简单,多列也可以比较,和单列一样。

案例:查询和SMITH的部门和岗位完全相同的所有雇员,不含SMITH本人

mysql> select ename, deptno, job from emp where (job, deptno) = (select job, deptno from emp where ename = 'SMITH') and ename <=> 'SMITH';
+-------+--------+-------+
| ename | deptno | job   |
+-------+--------+-------+
| SMITH |     20 | CLERK |
+-------+--------+-------+
1 row in set (0.00 sec)

1.3.4 在from子句中使用子查询

子查询语句出现在from子句中。这里要用到数据查询的技巧,把一个子查询当做一个临时表使用。

  • 显示每个高于自己部门平均工资的员工的姓名、部门、工资、平均工资
-- 先找到每个部门的平均工资
mysql> select deptno, avg(sal) from emp group by deptno;
+--------+-------------+
| deptno | avg(sal)    |
+--------+-------------+
|     10 | 2916.666667 |
|     20 | 2175.000000 |
|     30 | 1566.666667 |
+--------+-------------+
3 rows in set (0.00 sec)
-- 然后再用这张表和原本的表做笛卡尔积,多表查询
mysql> select emp.ename, emp.deptno, emp.sal, format(dept_avg,2) from emp , (select deptno , avg(sal) dept_avg from emp group by deptno) tmp wheree tmp.deptno = emp.deptno and emp.sal > dept_avg;
+-------+--------+---------+--------------------+
| ename | deptno | sal     | format(dept_avg,2) |
+-------+--------+---------+--------------------+
| ALLEN |     30 | 1600.00 | 1,566.67           |
| JONES |     20 | 2975.00 | 2,175.00           |
| BLAKE |     30 | 2850.00 | 1,566.67           |
| SCOTT |     20 | 3000.00 | 2,175.00           |
| KING  |     10 | 5000.00 | 2,916.67           |
| FORD  |     20 | 3000.00 | 2,175.00           |
+-------+--------+---------+--------------------+
6 rows in set (0.00 sec)
  • 显示每个部门的信息(部门名,编号,地址)和人员数量
    对于有未知的,聚合的信息(人员数量),先将它求出来
-- 首先拿到相关表的信息
mysql> select * from dept limit 3;
+--------+------------+----------+
| deptno | dname      | loc      |
+--------+------------+----------+
|     10 | ACCOUNTING | NEW YORK |
|     20 | RESEARCH   | DALLAS   |
|     30 | SALES      | CHICAGO  |
+--------+------------+----------+
3 rows in set (0.00 sec)

mysql> select * from emp limit 3;
+--------+-------+----------+------+---------------------+---------+--------+--------+
| empno  | ename | job      | mgr  | hiredate            | sal     | comm   | deptno |
+--------+-------+----------+------+---------------------+---------+--------+--------+
| 007369 | SMITH | CLERK    | 7902 | 1980-12-17 00:00:00 |  800.00 |   NULL |     20 |
| 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 |     30 |
| 007521 | WARD  | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 |     30 |
+--------+-------+----------+------+---------------------+---------+--------+--------+
3 rows in set (0.01 sec)
  • 暴力使用多表查询
    注意:和聚合函数一起显示的字段,需要被分组,因为聚合函数对于某一个字段是不清楚的,比如count(*),它是统计总数,并不对应某一个deptno,ename这些字段
mysql> select dept.dname, dept.deptno, dept.loc, count(*) from dept, emp where dept.deptno = emp.deptno group by deptno;
ERROR 1055 (42000): Expression #1 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'scott.dept.dname' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by
---- 注意:和聚合函数一起显示的字段,需要被分组,因为聚合函数对于某一个字段是不清楚的,比如count(*),它是统计总数,并不对应某一个deptno,ename这些字段
mysql> select dept.dname, dept.deptno, dept.loc, count(*) from dept, emp where dept.deptno = emp.deptno group by deptno, dept.dname, dept.loc;
+------------+--------+----------+----------+
| dname      | deptno | loc      | count(*) |
+------------+--------+----------+----------+
| ACCOUNTING |     10 | NEW YORK |        3 |
| RESEARCH   |     20 | DALLAS   |        5 |
| SALES      |     30 | CHICAGO  |        6 |
+------------+--------+----------+----------+
3 rows in set (0.00 sec)

  • 使用子查询
-- 先子查询出每个部门的人员数量
mysql> select deptno, count(*) from emp group by deptno;
+--------+----------+
| deptno | count(*) |
+--------+----------+
|     10 |        3 |
|     20 |        5 |
|     30 |        6 |
+--------+----------+
3 rows in set (0.00 sec)
-- 再和部门表组合,根据条件筛选。
mysql> select dept.dname, dept.deptno, dept.loc, dept_cnt from dept, (select deptno, count(*) dept_cnt from emp group by deptno) cnt_table where
cnt_table.deptno = dept.deptno;
+------------+--------+----------+----------+
| dname      | deptno | loc      | dept_cnt |
+------------+--------+----------+----------+
| ACCOUNTING |     10 | NEW YORK |        3 |
| RESEARCH   |     20 | DALLAS   |        5 |
| SALES      |     30 | CHICAGO  |        6 |
+------------+--------+----------+----------+
3 rows in set (0.00 sec)

1.4 合并查询

在实际应用中,为了合并多个select的执行结果,可以使用集合操作符 union,union all

1.4.1 union

该操作符用于取得两个结果集的并集。当使用该操作符时,会自动去掉结果集中的重复行

  • 将工资大于2500或职位是MANAGER的人找出来
    岗位是MANAGER的必定有工资大于2500的,这里自动去重了
mysql> select ename, sal, job from emp where sal>2500 union select ename, sal, job from emp where job='MANAGER';
+-------+---------+-----------+
| ename | sal     | job       |
+-------+---------+-----------+
| JONES | 2975.00 | MANAGER   |
| BLAKE | 2850.00 | MANAGER   |
| SCOTT | 3000.00 | ANALYST   |
| KING  | 5000.00 | PRESIDENT |
| FORD  | 3000.00 | ANALYST   |
| CLARK | 2450.00 | MANAGER   |
+-------+---------+-----------+
6 rows in set (0.00 sec)

1.4.2 union all

该操作符用于取得两个结果集的并集。当使用该操作符时,不会去掉结果集中的重复行

mysql> select ename, sal, job from emp where sal>2500 union all select ename, sal, job from emp where job='MANAGER';
+-------+---------+-----------+
| ename | sal     | job       |
+-------+---------+-----------+
| JONES | 2975.00 | MANAGER   |
| BLAKE | 2850.00 | MANAGER   |
| SCOTT | 3000.00 | ANALYST   |
| KING  | 5000.00 | PRESIDENT |
| FORD  | 3000.00 | ANALYST   |
| JONES | 2975.00 | MANAGER   |
| BLAKE | 2850.00 | MANAGER   |
| CLARK | 2450.00 | MANAGER   |
+-------+---------+-----------+
8 rows in set (0.00 sec)

二、内外连接

2.1 内连接

内连接inner join实际上就是利用where子句对两种表形成的笛卡儿积进行筛选,我们前面学习的查询都是内连接,也是在开发过程中使用的最多的连接查询。

select 字段 from1 inner join2 on 连接条件 and 其他条件;

前面使用的都是内连接。

on后面是两张表的连接条件,后可以跟where条件语句查询表。

-- 用前面的写法
select ename, dname from emp, dept where emp.deptno=dept.deptno and ename='SMITH';
-- 用标准的内连接写法
select ename, dname from emp inner join dept on emp.deptno=dept.deptno and ename='SMITH';

两张表连接,并不是所有行都是有效的,所有使用连接条件更加明确。

2.2 外连接

外连接分为左外连接和右外连接

2.2.1 左外连接

如果联合查询,左侧的表完全显示我们就说是左外连接。
什么叫完全显示呢?
就是两张表通过连接条件连接,但是左侧的表有不满足条件的也要全部显示,右侧的表显示为NULL。
语法:

select 字段名  from 表名1 left join 表名2 on 连接条件

创建两张表

mysql> select * from exam;
+------+-------+
| id   | grade |
+------+-------+
|    1 |    56 |
|    2 |    76 |
|   11 |     8 |
+------+-------+
3 rows in set (0.00 sec)

mysql> select * from stu;
+------+------+
| id   | name |
+------+------+
|    1 | jack |
|    2 | tom  |
|    3 | kity |
|    4 | nono |
+------+------+
4 rows in set (0.00 sec)
  • 使用内连接
mysql> select * from stu inner join exam on exam.id = stu.id;
+------+------+------+-------+
| id   | name | id   | grade |
+------+------+------+-------+
|    1 | jack |    1 |    56 |
|    2 | tom  |    2 |    76 |
+------+------+------+-------+
2 rows in set (0.00 sec)

  • 使用stu为左表的左外连接
mysql> select * from stu left join exam on exam.id = stu.id;
+------+------+------+-------+
| id   | name | id   | grade |
+------+------+------+-------+
|    1 | jack |    1 |    56 |
|    2 | tom  |    2 |    76 |
|    3 | kity | NULL |  NULL |
|    4 | nono | NULL |  NULL |
+------+------+------+-------+
4 rows in set (0.00 sec)
  • 使用exam为左表的左外连接
mysql> select * from exam left join stu on exam.id = stu.id;
+------+-------+------+------+
| id   | grade | id   | name |
+------+-------+------+------+
|    1 |    56 |    1 | jack |
|    2 |    76 |    2 | tom  |
|   11 |     8 | NULL | NULL |
+------+-------+------+------+
3 rows in set (0.00 sec)

2.2.1 右外连接

和左外连接对称,左侧不满足右侧的填充为NULL。

select 字段 from 表名1 right join 表名2  on 连接条件;
mysql> select * from exam right join stu on exam.id = stu.id;
+------+-------+------+------+
| id   | grade | id   | name |
+------+-------+------+------+
|    1 |    56 |    1 | jack |
|    2 |    76 |    2 | tom  |
| NULL |  NULL |    3 | kity |
| NULL |  NULL |    4 | nono |
+------+-------+------+------+
4 rows in set (0.00 sec)
  • 列出部门名称和这些部门的员工信息,同时列出没有员工的部门
    显然要以部门表为主
mysql> select dept.deptno, dept.dname, emp.ename, emp.job from dept left join emp on emp.deptno = dept.deptno;
+--------+------------+--------+-----------+
| deptno | dname      | ename  | job       |
+--------+------------+--------+-----------+
|     20 | RESEARCH   | SMITH  | CLERK     |
|     30 | SALES      | ALLEN  | SALESMAN  |
|     30 | SALES      | WARD   | SALESMAN  |
|     20 | RESEARCH   | JONES  | MANAGER   |
|     30 | SALES      | MARTIN | SALESMAN  |
|     30 | SALES      | BLAKE  | MANAGER   |
|     10 | ACCOUNTING | CLARK  | MANAGER   |
|     20 | RESEARCH   | SCOTT  | ANALYST   |
|     10 | ACCOUNTING | KING   | PRESIDENT |
|     30 | SALES      | TURNER | SALESMAN  |
|     20 | RESEARCH   | ADAMS  | CLERK     |
|     30 | SALES      | JAMES  | CLERK     |
|     20 | RESEARCH   | FORD   | ANALYST   |
|     10 | ACCOUNTING | MILLER | CLERK     |
|     40 | OPERATIONS | NULL   | NULL      |
+--------+------------+--------+-----------+
15 rows in set (0.00 sec)

左外连接是以左表为主
右外连接是以右表为主

你可能感兴趣的:(数据库,mysql,数据库,sql)