How Does WHERE Clause Placement Affect MySQL LEFT JOIN Efficiency?

MySQL Join with Where Clause

When joining two tables, it's possible to filter the results using a WHERE clause. However, the placement of this clause can significantly impact the efficiency of the join.

In the given case, the goal is to retrieve all categories and those subscribed to by a specific user. An initial query using a LEFT JOIN works well, but the addition of a WHERE clause on the user ID appears to convert it into an INNER JOIN.

To resolve this dilemma, it's crucial to understand the difference between join conditions and where clauses in outer joins. A join condition evaluates before the join operation and defines the rows that will be matched. A where clause, on the other hand, operates after the join and filters the resulting rowset.

The correct approach is to specify the filter in the join condition itself, as seen below:

SELECT *
FROM categories
LEFT JOIN user_category_subscriptions ON
  user_category_subscriptions.category_id = categories.category_id AND
  user_category_subscriptions.user_id = 1;

By placing the filter in the ON clause, only the matching rows from the user_category_subscriptions table will be joined to the categories table. This approach is more efficient and provides the desired result: all categories, including those subscribed to by the specific user.

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