一、连表查询

• 连接：将有联系的多张表通过关联(有联系就行，不一定是外键)字段，进行连接，形参一张大表
• 连表查询：在大表的基础上进行查询，就称之为连表查询
• 将表与表建立连接的方式有四种：内连接、左连接、右连接、全连接

数据准备

mysql>: 
create table dep(
    id int primary key auto_increment,
    name varchar(16),
    work varchar(16)
);
create table emp(
    id int primary key auto_increment,
    name varchar(16),
    salary float,
    dep_id int
);
insert into dep values(1, '市场部', '销售'), (2, '教学部', '授课'), (3, '管理部', '开车');
insert into emp(name, salary, dep_id) values('egon', 3.0, 2),('yanghuhu', 2.0, 2),('sanjiang', 10.0, 1),('owen', 88888.0, 2),('liujie', 8.0, 1),('yingjie', 1.2, 0);

笛卡尔积(基本没用)

# 笛卡尔积: 集合 X{a, b} * Y{o, p, q} => Z{{a, o}, {a, p}, {a, q}, {b, o}, {b, p}, {b, q}}

mysql>: select * from emp, dep;

# 总结：是两张表 记录的所有排列组合，数据没有利用价值

内连接

• 关键字：inner join on(inner可以省略)
• 语法：from A表 inner join B表 on A表.关联字段=B表.关联字段

mysql>: 
select emp.id,emp.name,salary,dep.name,work 
from dep inner join emp on dep.id=emp.dep_id;   # 内连接的inner可以省略

总结：只保留两个表有关联的数据

左连接

• 关键字：left join on
• 语法：from A表 left join B表 on A表.关联字段=B表.关联字段

mysql>: 
select 
    emp.id,emp.name,salary,dep.name,work 
from emp left join dep on emp.dep_id = dep.id 
order by emp.id;

总结：保留左表的全部数据，右表有对应数据直接连表显示，没有对应关系空填充

右连接

• 关键字：right join on
• 语法： from A表 left join B表 on A表.关联字段=B表.关联字段

mysql>: 
select 
    emp.id,emp.name,salary,dep.name,work 
from emp right join dep on emp.dep_id = dep.id 
order by emp.id;

总结：保留右表的全部数据，左表有对应数据直接连表显示，没有对应关系空填充

左右连接可以相互转化

mysql>: 
select 
    emp.id,emp.name,salary,dep.name,work 
from emp right join dep on emp.dep_id = dep.id 
order by emp.id;

mysql>: 
select 
    emp.id,emp.name,salary,dep.name,work 
from dep left join emp on emp.dep_id = dep.id 
order by emp.id;

总结：更换一下左右表的位置，相对应更换左右连接关键字，结果相同

全连接

将左连接和右连接通过关键字相连即实现全连接

关键字：union

mysql>: 
select 
    emp.id,emp.name,salary,dep.name,work 
from emp left join dep on emp.dep_id = dep.id 

union

select 
    emp.id,emp.name,salary,dep.name,work 
from emp right join dep on emp.dep_id = dep.id 
order by id;

总结：左表右表数据都被保留，彼此有对应关系正常显示，彼此没有对应关系均空填充对方

一对一与一对多情况一致

create table author(
    id int,
    name varchar(64),
    detail_id int
);
create table author_detail(
    id int,
    phone varchar(11)
);
insert into author values(1, 'Bob', 1), (2, 'Tom', 2), (3, 'ruakei', 2);
insert into author_detail values(1, '13344556677'), (2, '14466779988'), (3, '12344332255');

select author.id,name,phone from author join author_detail on author.detail_id = author_detail.id order by author.id;

select author.id,name,phone from author left join author_detail on author.detail_id = author_detail.id
union
select author.id,name,phone from author right join author_detail on author.detail_id = author_detail.id
order by id;

多对多

create table author(
    id int,
    name varchar(64),
    detail_id int
);
insert into author values(1, 'Bob', 1), (2, 'Tom', 2), (3, 'ruakei', 0);

create table book(
    id int,
    name varchar(64),
    price decimal(5,2)
);
insert into book values(1, 'python', 3.66), (2, 'Linux', 2.66), (3, 'Go', 4.66);

create table author_book(
    id int,
    author_id int,
    book_id int
);
# 数据：author-book：1-1,2  2-2,3  3-1,3
insert into author_book values(1,1,1),(2,1,2),(3,2,2),(4,2,3),(5,3,1),(6,3,3);

# 多对多
select book.name, book.price, author.name from book 
join author_book on book.id = author_book.book_id
join author on author_book.author_id = author.id;

# 多对多对1
select book.name, book.price, author.name, author_detail.phone from book 
join author_book on book.id = author_book.book_id
join author on author_book.author_id = author.id
left join author_detail on author.detail_id = author_detail.id;

二、子查询(重点***)

子查询：将一条查询sql的结果作为另一条sql的条件

• 增：insert into 表 select子查询结果
• 删：delete from 表 where条件 select子查询(查询的表不能与delete表相同)
• 查：select 字段 from 表 where条件 select子查询
• 改：update 表 set 字段=值 where select子查询(查询的表不能与update表相同)

# 子查询的sql
select dep, max(salary) from emp group by dep;
# 将子查询转换为一张表
create table t1(dep_name varchar(64), max_salary decimal(5,2));

## 子查询 - 增
insert into t1 select dep, max(salary) from emp group by dep;

## 子查询 - 删(查询的表不能与delete表相同)
delete from t1 where dep_name in (select distinct dep from emp);

## 子查询 - 改(查询的表不能与delete表相同)
update t1 set max_salary=max_salary+1 where dep_name in (select distinct dep from emp);

## 子查询 - 查
select * from emp where (dep, salary) in (select dep, max(salary) from emp group by dep);

三、all与any：区间修饰条件

# 语法规则
where id in (1, 2, 3) # id是1或2或3
where id not in (1, 2, 3) #  id不是1，2，3
where salary < all(3, 6, 9) # salary必须小于所有情况（小于最小）
where salary > all(3, 6, 9) # salary必须大于所有情况（大于最大）
where salary < any(3, 6, 9) # salary只要小于一种情况（小于最大）
where salary > any(3, 6, 9) # salary只要大于一种情况（大于最小）

# 案例，查询员工表中 薪资低于id大于11的员工薪资 所有的员工信息
select * from emp where salary < all(select salary from emp where id>11);

132 MySQL多表查询(重要)

原文：https://www.cnblogs.com/XuChengNotes/p/11595341.html