Home > Database > Mysql Tutorial > body text

Does MySQL's JOIN Default to LEFT JOIN?

Patricia Arquette
Release: 2024-11-12 12:56:01
Original
845 people have browsed it

Does MySQL's JOIN Default to LEFT JOIN?

Understanding the Distinction Between MySQL JOIN and LEFT JOIN

When constructing database queries, it's essential to understand the fundamental difference between JOIN and LEFT JOIN.

In the query provided, you've encountered an issue when replacing INNER JOINs with LEFT JOINs. This raises the question: is it assumed that a JOIN without a specified type defaults to LEFT JOIN?

No, it is not the case.

By default, MySQL performs INNER JOINs, which include only the rows that match in both tables. In contrast, LEFT JOINs preserve all rows from the left table (in your case, DM_Server.Jobs), even if they don't have corresponding matches in the right tables.

To illustrate this difference, let's consider the following:

  • INNER JOIN: Returns rows where both Jobs and servers have matching values for ServerID.
  • LEFT JOIN: Returns all rows from Jobs, including those that may not have corresponding entries in servers.

For a more comprehensive visual explanation:

INNER JOIN:

Jobs          Servers

| Jobs_ID | ServerID |
|---------|----------|
| 1        | 2        |
| 2        | 3        |

**Result:**
| Jobs_ID | ServerID | Description |
|---------|----------|------------|
| 1        | 2        | Description |
| 2        | 3        | Description |
Copy after login

LEFT JOIN:

Jobs          Servers

| Jobs_ID | ServerID |
|---------|----------|
| 1        | 2        |
| 2        | 3        |
| 3        | NULL     |

**Result:**
| Jobs_ID | ServerID | Description |
|---------|----------|------------|
| 1        | 2        | Description |
| 2        | 3        | Description |
| 3        | NULL     | NULL        |
Copy after login

By understanding this distinction, you can ensure that your queries retrieve the desired rows based on your intended join type.

The above is the detailed content of Does MySQL's JOIN Default to LEFT JOIN?. For more information, please follow other related articles on the PHP Chinese website!

source:php.cn
Statement of this Website
The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn
Latest Articles by Author
Popular Tutorials
More>
Latest Downloads
More>
Web Effects
Website Source Code
Website Materials
Front End Template