SQL left join and right join principle and example analysis

There are two tables, and the records in table A may not exist in table B.

Left join: focus on the left side, if there is no right side, it will be empty.
Right connection: focus on the right side, if there is no left side, it will be empty.
Inner Join: Returns the intersection

For example:

student table

id	name	age	class_id
1	yang	twenty two	1
2	su	20	1
3	fan	20	2
4	li	30	2
5	luo	twenty two

class table c

id	name	total
1	Freshman	30
2	Sophomore Year	15
3	Junior Year	40

In the table above, record number 5 in table s cannot be found in table c.

1. Left join: the left side of the left join is the main table, and if there is no corresponding secondary table, NULL is displayed.

SELECT s.`name`,s.`class_id` FROM student s LEFT JOIN class c ON s.`class_id`=c.`class_id`

result

name	class_id
yang	1
su	1
fan	2
li	2
luo	(NULL)

2. Right join: the right side of the right join is the primary table, and if there is no corresponding secondary table, NULL is displayed.

SELECT s.`name`,s.`class_id` FROM student s RIGHT JOIN class c ON s.`class_id`=c.`class_id`

result

name	class_id
yang	1
su	1
fan	2
li	2
(NULL)	(NULL)

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