多表查询
一：表的基本介绍
可以参考：
建立一个员工表信息表和一个部门表，每个员工都对应在哪个部门。因此这两张表具有一定关系。需要将两个表格进行查询，找到一个
员工所对应的所属部门。
01：建立一个员工表：employee 和一个部门表departmentemployee表：
mysql> create table employee(
-> id int primary key auto_increment,
-> name varchar(20),
-> sex enum("male","female") not null default "male",
-> age int,
-> dep_id int
-> );
Query OK, 0 rows affected (0.18 sec)
department表：
mysql> create table department(
-> id int,
-> name varchar(20)
-> );
Query OK, 0 rows affected (0.12 sec)
02：给两张表加入信息。
employee表：
mysql> insert into employee(name,sex,age,dep_id) values
-> ("egon","male",18,200),
-> ("alex","female",48,201),
-> ("wupeiqi","male",38,210),
-> ("yuanhao","female",28,202),
-> ("liwenzhou","female",28,202),
-> ("jingwen","female",18,204);
Query OK, 6 rows affected (0.05 sec)
Records: 6 Duplicates: 0 Warnings: 0
查看employee的结构：
mysql> desc employee;
+--------+-----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+-----------------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(20) | YES | | NULL | |
| sex | enum('male','female') | NO | | male | |
| age | int(11) | YES | | NULL | |
| dep_id | int(11) | YES | | NULL | |
+--------+-----------------------+------+-----+---------+----------------+
5 rows in set (0.00 sec)
查看employee的信息：
mysql> select * from employee;
+----+-----------+--------+------+--------+
| id | name | sex | age | dep_id |
+----+-----------+--------+------+--------+
| 1 | egon | male | 18 | 200 |
| 2 | alex | female | 48 | 201 |
| 3 | wupeiqi | male | 38 | 210 |
| 4 | yuanhao | female | 28 | 202 |
| 5 | liwenzhou | female | 28 | 202 |
| 6 | jingwen | female | 18 | 204 |
+----+-----------+--------+------+--------+
6 rows in set (0.00 sec)
department表：
mysql> insert into department values
-> (200,"技术"),
-> (201,"人力资源"),
-> (202,"销售"),
-> (203,"运营");
Query OK, 4 rows affected (0.04 sec)
Records: 4 Duplicates: 0 Warnings: 0
department的结构：
mysql> desc department;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
department的信息：
mysql> select * from department;
+------+--------------+
| id | name |
+------+--------------+
| 200 | 技术 |
| 201 | 人力资源 |
| 202 | 销售 |
| 203 | 运营 |
+------+--------------+
4 rows in set (0.00 sec)
二：多表查询（以employee和department为例）
#重点：外链接语法： select 字段列表 from 表1 inner/left/right join 表2 on 表1.字段=表2.字段；
01:交叉链接：生成笛卡儿积：
mysql> select * from employee,department;
+----+-----------+--------+------+--------+------+--------------+
| id | name | sex | age | dep_id | id | name |
+----+-----------+--------+------+--------+------+--------------+
| 1 | egon | male | 18 | 200 | 200 | 技术 |
| 1 | egon | male | 18 | 200 | 201 | 人力资源 |
| 1 | egon | male | 18 | 200 | 202 | 销售 |
| 1 | egon | male | 18 | 200 | 203 | 运营 |
| 2 | alex | female | 48 | 201 | 200 | 技术 |
| 2 | alex | female | 48 | 201 | 201 | 人力资源 |
| 2 | alex | female | 48 | 201 | 202 | 销售 |
| 2 | alex | female | 48 | 201 | 203 | 运营 |
| 3 | wupeiqi | male | 38 | 210 | 200 | 技术 |
| 3 | wupeiqi | male | 38 | 210 | 201 | 人力资源 |
| 3 | wupeiqi | male | 38 | 210 | 202 | 销售 |
| 3 | wupeiqi | male | 38 | 210 | 203 | 运营 |
| 4 | yuanhao | female | 28 | 202 | 200 | 技术 |
| 4 | yuanhao | female | 28 | 202 | 201 | 人力资源 |
| 4 | yuanhao | female | 28 | 202 | 202 | 销售 |
| 4 | yuanhao | female | 28 | 202 | 203 | 运营 |
| 5 | liwenzhou | female | 28 | 202 | 200 | 技术 |
| 5 | liwenzhou | female | 28 | 202 | 201 | 人力资源 |
| 5 | liwenzhou | female | 28 | 202 | 202 | 销售 |
| 5 | liwenzhou | female | 28 | 202 | 203 | 运营 |
| 6 | jingwen | female | 18 | 204 | 200 | 技术 |
| 6 | jingwen | female | 18 | 204 | 201 | 人力资源 |
| 6 | jingwen | female | 18 | 204 | 202 | 销售 |
| 6 | jingwen | female | 18 | 204 | 203 | 运营 |
+----+-----------+--------+------+--------+------+--------------+
24 rows in set (0.00 sec)
这种显示是将所有列表排序都生成了，我们要找的排序肯定会在这个笛卡儿积表格中，但是不利于分析。
View Code
02：内链接，只链接匹配的行。
mysql> select * from employee,department where department.id=employee.dep_id; #where条件分析。
+----+-----------+--------+------+--------+------+--------------+
| id | name | sex | age | dep_id | id | name |
+----+-----------+--------+------+--------+------+--------------+
| 1 | egon | male | 18 | 200 | 200 | 技术 |
| 2 | alex | female | 48 | 201 | 201 | 人力资源 |
| 4 | yuanhao | female | 28 | 202 | 202 | 销售 |
| 5 | liwenzhou | female | 28 | 202 | 202 | 销售 |
+----+-----------+--------+------+--------+------+--------------+
4 rows in set (0.00 sec)
View Code
#再看一个需求，我要查出技术部的员工的名字
mysql> select name from employee,department where employee.dep_id=department.id and department.name='技术';
ERROR 1052 (23000): Column 'name' in field list is ambiguous
#上面直接就报错了，因为select后面直接写的name，在两个表合并起来的表中，是有两个name字段的，直接写name是不行的，要加上表名，再看：
mysql> select employee.name from employee,department where employee.dep_id=department.id and department.name="技术";
+------+
| name |
+------+
| egon |
+------+
1 row in set (0.00 sec)
03:外链接之左链接：优先显示左边表的全部记录
#以左表为准，即找出所有员工信息，当然包括没有部门的员工
#本质就是：在内链接的基础上增加左边有而右边没有的结果
mysql> select employee.id,employee.name,department.name as depart_name from employee
> left join department on employee.dep_id=department.id;
+----+-----------+--------------+
| id | name | depart_name |
+----+-----------+--------------+
| 1 | egon | 技术 |
| 2 | alex | 人力资源 |
| 4 | yuanhao | 销售 |
| 5 | liwenzhou | 销售 |
| 3 | wupeiqi | NULL |
| 6 | jingwen | NULL |
+----+-----------+--------------+
6 rows in set (0.00 sec)
04:外链接之右链接：优先显示右边表的全部记录
#以右表为准，即找出所有员工信息，当然包括没有部门的员工
#本质就是：在内链接的基础上增加右边有而左边没有的结果
mysql> select employee.id,employee.name,department.name as depart_name from
-> employee right join department on
-> employee.dep_id=department.id;
+------+-----------+--------------+
| id | name | depart_name |
+------+-----------+--------------+
| 1 | egon | 技术 |
| 2 | alex | 人力资源 |
| 4 | yuanhao | 销售 |
| 5 | liwenzhou | 销售 |
| NULL | NULL | 运营 |
+------+-----------+--------------+
5 rows in set (0.00 sec)
05:全外链接：显示左右两个表的全部内容
001：union显示的左右两边的数据，并将重复的数据去重。
mysql> select * from employee left join department on employee.dep_id=department.id
-> union
-> select * from employee right join department on employee.dep_id=department.id
-> ;
+------+-----------+--------+------+--------+------+--------------+
| id | name | sex | age | dep_id | id | name |
+------+-----------+--------+------+--------+------+--------------+
| 1 | egon | male | 18 | 200 | 200 | 技术 |
| 2 | alex | female | 48 | 201 | 201 | 人力资源 |
| 4 | yuanhao | female | 28 | 202 | 202 | 销售 |
| 5 | liwenzhou | female | 28 | 202 | 202 | 销售 |
| 3 | wupeiqi | male | 38 | 210 | NULL | NULL |
| 6 | jingwen | female | 18 | 204 | NULL | NULL |
| NULL | NULL | NULL | NULL | NULL | 203 | 运营 |
+------+-----------+--------+------+--------+------+--------------+
7 rows in set (0.06 sec)
View Code
002：union all 显示的是左右两边表的数据，并不会去重
mysql> select * from employee left join department on employee.dep_id=department.id
-> union all
-> select * from employee right join department on employee.dep_id=department.id;
+------+-----------+--------+------+--------+------+--------------+
| id | name | sex | age | dep_id | id | name |
+------+-----------+--------+------+--------+------+--------------+
| 1 | egon | male | 18 | 200 | 200 | 技术 |
| 2 | alex | female | 48 | 201 | 201 | 人力资源 |
| 4 | yuanhao | female | 28 | 202 | 202 | 销售 |
| 5 | liwenzhou | female | 28 | 202 | 202 | 销售 |
| 3 | wupeiqi | male | 38 | 210 | NULL | NULL |
| 6 | jingwen | female | 18 | 204 | NULL | NULL |
| 1 | egon | male | 18 | 200 | 200 | 技术 |
| 2 | alex | female | 48 | 201 | 201 | 人力资源 |
| 4 | yuanhao | female | 28 | 202 | 202 | 销售 |
| 5 | liwenzhou | female | 28 | 202 | 202 | 销售 |
| NULL | NULL | NULL | NULL | NULL | 203 | 运营 |
+------+-----------+--------+------+--------+------+--------------+
11 rows in set (0.00 sec)
View Code
三：符合条件接连的查询
1、
#示例1：以内连接的方式查询employee和department表，并且employee表中的age字段值必须大于25,即找出年龄大于25岁的员工以及员工所在的部门
mysql> select employee.name,department.name from employee inner join department
-> on employee.dep_id=department.id
-> where age>25;
+-----------+--------------+
| name | name |
+-----------+--------------+
| alex | 人力资源 |
| yuanhao | 销售 |
| liwenzhou | 销售 |
+-----------+--------------+
3 rows in set (0.00 sec)
2、#实例：以内连接的方式查询employee和department表，并且以age字段的生序方式显示；
实例01：内连接的方式查询employee和department表，并且employee表中的age字段值必须大于25,即找出年龄大于25岁的员工以及员工所在的部门
mysql> select employee.name,department.name from employee inner join department
-> on employee.dep_id=department.id where age>25
-> order by age; #(order by 排序是默认的升序的，升序order by asc)
+-----------+--------------+
| name | name |
+-----------+--------------+
| yuanhao | 销售 |
| liwenzhou | 销售 |
| alex | 人力资源 |
+-----------+--------------+
3 rows in set (0.00 sec)
实例一
实例02：以内连接的方式查询employee和department表，并且以age字段的升序方式显示
mysql> select employee.id,employee.name,employee.age,department.name from employee,department
-> where employee.dep_id=department.id
-> and age >25
-> order by age asc;
+----+-----------+------+--------------+
| id | name | age | name |
+----+-----------+------+--------------+
| 5 | liwenzhou | 28 | 销售 |
| 4 | yuanhao | 28 | 销售 |
| 2 | alex | 48 | 人力资源 |
+----+-----------+------+--------------+
3 rows in set (0.00 sec)
实例二
四：子查询
解释：子查询就是将一个查询结果用括号括起来，交给另外一个sql语句，作为它的一个查询语句来进行操作。
子查询：
#1：子查询是将一个查询语句嵌套在另一个查询语句中。
#2：内层查询语句的查询结果，可以为外层查询语句提供查询条件。
#3：子查询中可以包含：IN、NOT IN、ANY、ALL、EXISTS 和 NOT EXISTS等关键字
#4：还可以包含比较运算符：= 、 !=、> 、<等
1: 带in关键字的的字查询：
mysql> select id,name from department where id in (select dep_id from employee group by
-> dep_id having avg(age)>25);
+------+--------------+
| id | name |
+------+--------------+
| 201 | 人力资源 |
| 202 | 销售 |
+------+--------------+
2 rows in set (0.05 sec)
总结：子查询的思路和解决问题一样，先解决一个然后拿着这个的结果再去解决另外一个问题，连表的思路是先将两个表关联在一起，
然后在进行group by啊过滤啊等等操作，两者的思路是不一样的.
01:#查询员工平均年龄在25岁以上的部门名，可以用连表，也可以用子查询，
mysql> select department.name from department inner join employee on department.id=employee.dep_id
-> group by department.name having avg(age)>25;
+--------------+
| name |
+--------------+
| 人力资源 |
| 销售 |
+--------------+
案例01
02:#查看技术部员工姓名
mysql> select name from employee where dep_id in (select id from department where name="技术");
+------+
| name |
+------+
| egon |
+------+
1 row in set (0.00 sec)
案例02
03:#查看不足1人的部门名(子查询得到的是有人的部门id)
mysql> select id,name from department where id not in (select distinct dep_id from employee);
+------+--------+
| id | name |
+------+--------+
| 203 | 运营 |
+------+--------+
1 row in set (0.00 sec)
案例03
2:比较运算符的子查询：
#比较运算符：=、!=、>、>=、<、<=、<>
#查询大于所有人平均年龄的员工名与年龄
mysql> select name,age from employee where age >(select avg(age) from employee);
+---------+------+
| name | age |
+---------+------+
| alex | 48 |
| wupeiqi | 38 |
+---------+------+
2 rows in set (0.00 sec)
3：带exists(存在）关键字的子查询
EXISTS关字键字表示存在。在使用EXISTS关键字时，内层查询语句不返回查询的记录。而是返回一个真假值。True或False
当返回True时，外层查询语句将进行查询；当返回值为False时，外层查询语句不进行查询。还可以写not exists，和exists的效果就是反的
01：employee表中不存在id为200
mysql> select * from employee where exists (select id from employee where id=500);
Empty set (0.00 sec)
02：employee存在的情况：
#只有当括号里面的条件成立了，select * from employee 才会执行
mysql> select * from employee where exists (select dep_id from employee where dep_id=200);
+----+-----------+--------+------+--------+
| id | name | sex | age | dep_id |
+----+-----------+--------+------+--------+
| 1 | egon | male | 18 | 200 |
| 2 | alex | female | 48 | 201 |
| 3 | wupeiqi | male | 38 | 210 |
| 4 | yuanhao | female | 28 | 202 |
| 5 | liwenzhou | female | 28 | 202 |
| 6 | jingwen | female | 18 | 204 |
+----+-----------+--------+------+--------+
6 rows in set (0.00 sec)
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