Why are My GROUP_CONCAT Results Duplicated After Adding LEFT JOINs?

GROUP_CONCAT produces duplicate values: problems after adding LEFT JOIN

Question:

A user has duplicate values ​​in their GROUP_CONCAT results after adding a category LEFT JOIN to an existing query that retrieves user data, tags, and reputation.

Problem description:

The original query correctly retrieves the first two tags associated with each user. However, after adding the category LEFT JOIN, there are some duplicates in the results for labels and categories. The expected output is that the first two categories should be unique for each user.

Solution:

The problem occurs because the LEFT JOIN causes each user_id to be joined with multiple combinations of tags and categories, resulting in duplicate rows after the GROUP BY. In order to solve this problem, several methods can be used:

Cumulative LEFT JOIN method:

• Perform a LEFT JOIN between the original query and the tag table.
• Group By the results and perform GROUP_CONCAT.
• Perform a LEFT JOIN between the cumulative results and the category table.
• Group By the category column and perform GROUP_CONCAT.

Symmetric Inner Join method:

• Perform a LEFT JOIN between the original query and the tag table.
• Group By the results and perform GROUP_CONCAT.
• Perform a LEFT JOIN separately between the original query and the category table.
• Group By the results and perform GROUP_CONCAT.
• Inner Join the two results based on user_id.

Scalar subquery method:

• Extract GROUP_CONCAT results for labels and categories separately using scalar subqueries, each containing a GROUP BY.

Modified LEFT JOIN method using DISTINCT GROUP_CONCAT:

• Perform a LEFT JOIN between the original query and the tag table.
• Group By the result and perform GROUP_CONCAT using DISTINCT.
• Perform a LEFT JOIN between the results and category tables.
• Group By the category column and perform GROUP_CONCAT using DISTINCT.

The choice of approach may depend on engineering trade-offs, including query plan efficiency, data duplication levels, and timing.

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