Mysql

内连接、外连接

• 创建一个学生表

create table students
(
    id int primary key auto_increment,
    name varchar(100)
);

• 创建一个成绩表

create table scores
(
   scoreId int primary key auto_increment,
   stuId int not null,
   math double
);

• 给 两张表插入数据

insert into students values(null, "smx");
insert into students values(null, "xyj");
insert into students values(null, "wdd");
insert into students values(null, "hhr");

insert into scores values(null, 1, 100);
insert into scores values(null, 2, 99);
insert into scores values(null, 3, 98);
insert into scores values(null, 5, 97);

image.png
image.png

• 内连接

select * from students as stu inner join scores as s on stu.id = s.stuId; # 内连接
select * from students as stu join scores as s on stu.id = s.stuId; # 内连接

image.png

• 左连接
  select * from students as stu left join scores as s on stu.id = s.stuId; # 左连接
  

  image.png

• 右连接
  ``select * from students as stu right join scores as s on stu.id = s.stuId; # 右连接

  
  image.png

• 外连接

select * from students as stu left join scores as s on stu.id = s.stuId
union
select * from students as stu right join scores as s on stu.id = s.stuId;  # mysql没有全外连接的实现，只能自己手动实现，取左连接和右连接的并集

image.png