MySql8.0(05)

MySql8.0's supplement to multi-table design

Internal connection

Equivalent connection

 -- 等值连接
 -- 语法 select 结果 from 表1 inner join 表2  on 连接条件
 SELECT s.num,s.name,s.sex,g.name
 FROM student s
 INNER JOIN grade g
 ON s.num = g.id

-- 写法二
 SELECT s.num,s.name,s.sex,g.name
 FROM student s,grade g
 WHERE s.num = g.id

Non-equivalent join

-- 非等值连接
 CREATE TABLE score_level(
	NAME VARCHAR(1),
	lower_score INT,
	upper_score INT
 )
 
 SELECT s.num,s.name,sl.name
 FROM student s
 INNER JOIN score_level sl
 ON s.score BETWEEN sl.lower_score AND sl.upper_score

Self-connection

 -- 自连接
 CREATE TABLE t_area(
	id INT,
	NAME VARCHAR(20),
	pid INT
 )
 
 SELECT t1.id,t1.name,t2.id,t2.name
 FROM t_area t1 
 INNER JOIN t_area t2
 ON t1.pid = t2.id

Outer join

Left outer join

 -- 左外连接(以左边的表为中心查询,无论条件是否满足都会将左边数据查询出来) 
 -- 学号,姓名,性别,年级名次
 SELECT s.num,s.name,s.sex,g.name
 FROM student s 
 LEFT JOIN grade g
 ON s.grade = g.id

Right outer join

  -- 右外连接  学号,姓名,性别,年级名次
 SELECT s.num,s.name,s.sex,g.name
 FROM student s 
 RIGHT JOIN grade g
 ON s.grade = g.id
 

-Use the sqlyog client tool to back up the database data
- Back up the data before modifying or deleting data