SQL - Left join, Right join, Inner join

Article Directory

• Foreword
• I. Concepts
• Second, Example
• Summary

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Foreword

I have been doing SQL-related exercises recently, and I found that the SQL I used to be so confident has become tricky if I don't touch it for a long time, especially the internal and external connections between this table.So this article is about sorting out this internal and external connection.

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I. Concept

First of all, let's introduce the definitions of these three

1.Left join: that is, left join, which is based on the left table, and connects the two tables according to the conditions of the two tables given after ON.As a result, all the query information in the left table will be listed, while the right table will only list the parts that satisfy the condition after ON and the left table.Left join is called left outer join, which is a type of outer join.

2.Right join: The right join is based on the right table, and connects the two tables according to the conditions of the two tables given after ON.As a result, all the query information in the right table will be listed, while the left table will only list the parts that satisfy the condition after ON and the right table.Right join is called right outer join, which is a kind of outer join.

3.Inner join: that is, inner join, using the two tables as reference objects at the same time, and connecting the two tables according to the conditions of the two tables given after ON.As a result, only the parts of the two tables that satisfy the ON condition will be listed.

There is no full outer join in MySQL, so it is not explained here.

2. Examples

Table table_a

table table_b

----------------------------------------------------------------Dividing line-----------------------------------------------------------------

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Left join: left join

SQL Statement:

select *from table_a left join table_b on table_a.id=table_b.id;

Results:

Right join: right join

SQL Statement:

select * from table_a right join table_b on table_a.id=table_b.id;

Result:

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Inner join: Inner join

SQL Statement:

select * from table_a inner join table_b on table_a.id=table_b.id;

Result:

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Summary

Based on the chestnuts above, it looks much clearer and more contrasting.If you still think that you only understand it now, but you still find it a little difficult to use, you can use the following collective thinking, which I have only recently discovered, and it will be much simpler to think about it this way.

We have two tables: A and B, then treat them as a set respectively

Then our left and right outer joins and inner joins are equivalent to this ↓↓↓

A Left join B-> A (according to the return requirement, it needs to connect the part of B with A and other conditions)

A Right join B -> B (according to the return requirements, the parts of A and B need to be connected)

A Inner join B -> A∩B (the part that needs to be connected with conditions such as A and B according to the return requirement)

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