What's the Difference Between Inner, Outer, Left, and Right Joins in SQL?

In-depth understanding of inner joins, outer joins, left joins and right joins in SQL

It is crucial to be proficient in the differences between various SQL connection types. This guide explains in detail the characteristics of inner joins, outer joins, left joins, and right joins.

Inner Join

Inner joins only return rows that match in both tables. This means that if a row does not have a matching row in another table, it will be excluded from the results.

Outer Join

Contrary to inner joins, outer joins include rows from one or both tables even if there are no matches in the other table. There are two types of outer joins:

• Left Outer Join: Returns all rows in the left table regardless of whether there is a match in the right table. Null values ​​indicate missing data in non-matching rows.
• Right Outer Join: Similar to Left Outer Join, but returns all rows from the right table and fills null values ​​in unmatched rows from the left table.

Performance Considerations

The performance of a join depends on several factors, including the size of the table, the number of columns involved, and the type of join used. In general, inner joins are the most efficient because they produce the smallest number of rows. Outer joins, on the other hand, can be more expensive because they contain extra rows that may not be needed.

Reference materials

For a more detailed explanation and visual representation of the different connection types, please see the following article:

Visual explanation of SQL connections

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