How to Avoid Duplicate Values in GROUP_CONCAT after Multiple LEFT JOINs of Grouped Queries?

Troubleshooting Duplicate Values in GROUP_CONCAT after Multiple LEFT JOINs on Grouped Queries

Using GROUP_CONCAT with multiple LEFT JOINs on grouped queries can lead to duplicate values. This often happens when a third LEFT JOIN introduces multiple rows for each (tag, category) combination linked to a user ID. The subsequent GROUP BY then produces duplicate (user ID, tag) and (user ID, category) pairs within the GROUP_CONCAT results.

Several strategies can address this duplication:

1. Employing DISTINCT within GROUP_CONCAT: Adding DISTINCT inside the GROUP_CONCAT function directly removes duplicate values within each concatenated list.

2. Sequential LEFT JOIN Approach: Perform LEFT JOINs sequentially. First, join the user table with the tags table, group the results, and then LEFT JOIN this intermediate result with the categories table, grouping again. This ensures each user ID has a single row with distinct tag and category combinations.

3. Leveraging Scalar Subqueries: Utilize scalar subqueries within the SELECT clause to calculate GROUP_CONCAT values independently for each user ID, guaranteeing a single row per user.

4. Utilizing Symmetrical INNER JOINs: Instead of LEFT JOINs, employ INNER JOINs. Join the user table with the tags table, then perform another INNER JOIN based on the user ID. Repeat this process for the user table and categories table.

The optimal solution depends on the data structure and query performance. Analyzing query plans and execution times is crucial for selecting the most efficient method.

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