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:
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.
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!