MySQL多表查询

一、SELECT语句关键字的执行顺序

     ⑦ select

     ⑧ distinct <select_list>     #去除重复

     ① from <left_table>　

     ③ <join_type> join <right_table>

     ② on <join_condition>

     ④ where <where_condition>

     ⑤ group by <group_by_list>

     ⑥ having <having_condition>

     ⑨ order by <order_by_condition>

     ⑩ limit <limit_number>

二、多表连接查询

#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;

准备表数据

#建表
create table dep(
id int,
name varchar(20) 
);

create table emp(
id int primary key auto_increment,
name varchar(20),
sex enum('male','female') not null default 'male',
age int,
dep_id int
);

#插入数据
insert into dep values
(200,'技术'),
(201,'人力资源'),
(202,'销售'),
(203,'运营');

insert into emp(name,sex,age,dep_id) values
('egon','male',18,200),
('alex','female',48,201),
('wupeiqi','male',38,201),
('yuanhao','female',28,202),
('liwenzhou','male',18,200),
('jingliyang','female',18,204)
;

#查看表结构和数据
mysql> desc dep;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+

mysql> desc emp;
+--------+-----------------------+------+-----+---------+----------------+
| 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 | |
+--------+-----------------------+------+-----+---------+----------------+

mysql> select * from dep;
+------+--------------+
| id | name |
+------+--------------+
| 200 | 技术 |
| 201 | 人力资源 |
| 202 | 销售 |
| 203 | 运营 |
+------+--------------+

mysql> select * from emp;
+----+------------+--------+------+--------+
| id | name | sex | age | dep_id |
+----+------------+--------+------+--------+
| 1 | egon | male | 18 | 200 |
| 2 | alex | female | 48 | 201 |
| 3 | wupeiqi | male | 38 | 201 |
| 4 | yuanhao | female | 28 | 202 |
| 5 | liwenzhou | male | 18 | 200 |
| 6 | jingliyang | female | 18 | 204 |
+----+------------+--------+------+--------+

1、交叉连接：不适用任何匹配条件。生成笛卡尔积

mysql> select * from emp,dep;

2、内连接：只连接匹配的行

mysql> select * from emp inner join dep on emp.emp_id = dep.id;

3、外链接之左连接：优先显示左表全部记录

mysql> select * from emp left join dep on emp.emp_id = dep.id;

4、外链接之右连接：优先显示右表全部记录

mysql> select * from emp right join dep on emp.emp_id = dep.id;

5、全外连接：显示左右两个表全部记录

全外连接：在内连接的基础上增加左边有右边没有的和右边有左边没有的结果
#注意：mysql不支持全外连接 full join
#强调：mysql可以使用此种方式间接实现全外连接
mysql> select * from emp left join dep on emp.dep_id = dep.id union select * from emp right join dep on emp.dep_id = dep.id;
+------+------------+--------+------+--------+------+--------------+
| id   | name       | sex    | age  | dep_id | id   | name         |
+------+------------+--------+------+--------+------+--------------+
|    1 | egon       | male   |   18 |    200 |  200 | 技术         |
|    5 | liwenzhou  | male   |   18 |    200 |  200 | 技术         |
|    2 | alex       | female |   48 |    201 |  201 | 人力资源     |
|    3 | wupeiqi    | male   |   38 |    201 |  201 | 人力资源     |
|    4 | yuanhao    | female |   28 |    202 |  202 | 销售         |
|    6 | jingliyang | female |   18 |    204 | NULL | NULL         |
| NULL | NULL       | NULL   | NULL |   NULL |  203 | 运营         |
+------+------------+--------+------+--------+------+--------------+

#注意 union与union all的区别：union会去掉相同的纪录

三、条件匹配查询

#示例1：以内连接的方式查询emp和dep表，并且emp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
select emp.name,dep.name from emp inner join dep on emp.dep_id = dep.id where age > 25;

#示例2：以where过滤的方式查询emp和dep表，并且以age字段的升序方式显示
select emp.id,emp.name,emp.age,dep.name from emp,dep where emp.dep_id = dep.id and age > 25 order by age asc;

四、合并两次查询结果

SELECT * FROM (SELECT * FROM `testtable` WHERE name IS NOT NULL) AS temp UNION ALL (SELECT * FROM `testtable` WHERE name IS NULL) ORDER BY id DESC LIMIT 0,10;
SELECT * FROM (SELECT * FROM `domain` WHERE nginx=1 AND addr="www") AS temp UNION ALL (SELECT * FROM `domain` WHERE nginx=2) ORDER BY id DESC LIMIT 0,10;

五、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关键字的子查询

#查询平均年龄在25岁以上的部门名
select id,name from dep
    where id in 
        (select dep_id from emp group by dep_id having avg(age) > 25);

#查看技术部员工姓名
select name from emp
    where dep_id in 
        (select id from dep where name='技术');

#查看不足1人的部门名
select name from dep
    where id in 
        (select dep_id from emp group by dep_id having count(id) <=1);

2、带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 emp where age > (select avg(age) from emp);
+---------+------+
| name | age |
+---------+------+
| alex | 48 |
| wupeiqi | 38 |
+---------+------+
rows in set (0.00 sec)

#查询大于部门内平均年龄的员工名、年龄
select t1.name,t1.age from emp t1
  inner join 
    (select dep_id,avg(age) avg_age from emp group by dep_id) t2
      on t1.dep_id = t2.dep_id
        where t1.age > t2.avg_age;

3、带exists关键字的子查询
EXISTS关键字后面的参数可以是任意一个子查询，它不产生任何数据只返回TRUE或FALSE。当返回值为TRUE时外层查询才会执行

#department表中存在dept_id=203，Ture
mysql> select * from emp where exists(select id from dep where id=200);
+----+------------+--------+------+--------+
| id | name       | sex    | age  | dep_id |
+----+------------+--------+------+--------+
|  1 | egon       | male   |   18 |    200 |
|  2 | alex       | female |   48 |    201 |
|  3 | wupeiqi    | male   |   38 |    201 |
|  4 | yuanhao    | female |   28 |    202 |
|  5 | liwenzhou  | male   |   18 |    200 |
|  6 | jingliyang | female |   18 |    204 |
+----+------------+--------+------+--------+

#dep表中存在dept_id=205，False
mysql> select * from emp where exists(select id from dep where id=204);
Empty set (0.00 sec)

4、带ANY关键字的子查询
ANY关键字表示满足其中任意一个条件就返回一个结果作为外层查询条件。

# 查询比任一部门id号还大的人员信息
mysql> select * from emp where dep_id > any (select id from dep);
+----+------------+--------+------+--------+
| id | name       | sex    | age  | dep_id |
+----+------------+--------+------+--------+
|  2 | alex       | female |   48 |    201 |
|  3 | wupeiqi    | male   |   38 |    201 |
|  4 | yuanhao    | female |   28 |    202 |
|  6 | jingliyang | female |   18 |    204 |
+----+------------+--------+------+--------+

5、带ALL关键字的子查询
ALL关键字与ANY有点类似，只不过带ALL关键字的子査询返回的结果需同时满足所有内层査询条件。

# 查询比所有部门id号还大的人员信息
mysql> select * from emp where dep_id > all (select id from dep);
+----+------------+--------+------+--------+
| id | name       | sex    | age  | dep_id |
+----+------------+--------+------+--------+
|  6 | jingliyang | female |   18 |    204 |
+----+------------+--------+------+--------+ 

六、联表更新 

# 联表更新：(engineer e)重命名engineer表为e，(project p)project表为p
UPDATE engineer e INNER JOIN project p ON e.`pid` = p.`id` SET e.username="小明",e.ctime=NOW(),e.status=1 WHERE p.title="test";
UPDATE fee_item f,sign s SET f.start_date ="2020-01-01",s.status =1  WHERE f.code = s.code AND  s.merchant_id = "10";

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