How to Correctly Use GROUP_CONCAT with LEFT JOIN in MySQL to Concatenate Solutions per Ticket?

MySQL GROUP_CONCAT with LEFT JOIN Issue and Solution

In MySQL, the GROUP_CONCAT function is commonly used to concatenate strings from multiple rows in a group. However, when utilizing this function with a LEFT JOIN, it's possible to encounter a situation where multiple rows are combined into a single row instead of being separated.

Problem Statement

Consider the following scenario:

• Two tables, "Tickets" and "Solutions," representing a help desk database
• Each ticket has one or more corresponding solutions
• Goal: Create a query that combines all solutions for each ticket

The query being used:

<code class="SQL">SELECT Tickets.*, GROUP_CONCAT(Solutions.solution) AS CombinedSolutions
FROM Tickets
LEFT JOIN Solutions ON Tickets.id = Solutions.ticket_id
ORDER BY Tickets.id;</code>

Issue

This query returns only one row, with ticket 1's information and both ticket 1's and ticket 2's solution entries concatenated together.

Solution

To resolve this issue, use a subquery or derived table within the JOIN statement to group the solutions by ticket ID:

Solution 1: Using Subquery

<code class="SQL">SELECT t.*,
          x.combinedsolutions
     FROM TICKETS t
LEFT JOIN (SELECT s.ticket_id,
                  GROUP_CONCAT(s.solution) AS combinedsolutions
             FROM SOLUTIONS s 
         GROUP BY s.ticket_id) x ON x.ticket_id = t.ticket_id</code>

Solution 2: Using Derived Table

<code class="SQL">SELECT t.*,
          (SELECT GROUP_CONCAT(s.solution)
             FROM SOLUTIONS s 
            WHERE s.ticket_id = t.ticket_id) AS combinedsolutions
     FROM TICKETS t</code>

By grouping the solutions before joining, the query now returns separate rows for each ticket, with their corresponding solutions concatenated into a single string.

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