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 |

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        |

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

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