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

class table c

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

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

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