Tips for using the SQL LEFT JOIN and Right Join _ database other

Here is an example to illustrate:

Cases
Table A

Aid Adate
1 A1
2 A2
3 A3

Table B

Bid Bdate
1 B1
2 B2
4 B4

Left JOIN:

SELECT * from a LEFT join B on a.aid = B.bid

First remove all the data from table A and then add the data that matches the A,b
At this point, the removal is:

1 A1 B1
2 A2 B2
3 A3 NULL characters

There is also the right join

It means to first remove all the data from table B and then add the data that matches the a,b.
At this point, the removal is:

1 A1 B1
2 A2 B2
4 NULL character B4

Oh, we must note that the left and right connection with two table a,b connected, to remove the same ID field data is not the same,
SELECT * from A, b where a.aid = B.bid This is to remove only matching data.
At this point, the removal is:
1 A1 B1
2 A2 B2

Here, by the way, the Union, union all.

Union is the data that connects two tables and excludes all duplicate data.

The Union all does not exclude duplicate data.

Here are some of the articles you might be interested in:
Super detail MySQL left join,right join,inner join usage Analysis
SQL External link Action summary inner join LEFT JOIN right Join
Left join RIGHT join in SQL