How to Join MySQL Tables Based on IDs Stored in a JSON Field?

Using MySQL JSON Field for Joining Tables

Storing data in a JSON field can provide flexibility, but it also presents challenges when performing data operations. In this inquiry, the objective is to join a table using a JSON field that stores an array of IDs.

Problem Statement

The user has created two tables, user and user_group, and a fictitious user_groups JSON field within the user table to store a list of IDs. The user wants to perform operations on this JSON field and use them in SQL.

Specifically, the user seeks to obtain results that list user IDs, user names, user group IDs, and group names. These results should be joined based on the IDs stored in the JSON field.

Solution

To achieve this, the user can leverage the JSON_CONTAINS function. This function enables users to check if a JSON document contains a specific value or subquery.

Code

<code class="sql">SELECT 
       u.user_id, 
       u.user_name, 
       g.user_group_id,
       g.group_name
   FROM user u
   LEFT JOIN user_group g on JSON_CONTAINS(u.user_groups, CAST(g.user_group_id as JSON), '$')</code>

Explanation

• The JSON_CONTAINS function checks if the user_groups JSON field of each row in the user table contains the corresponding user_group_id from the user_group table. It uses $ to indicate the top-level object in the JSON document.
• The CAST expression converts the user_group_id to a JSON string before comparing it to the JSON field.
• The result of the comparison is used to perform a left join between the user and user_group tables, ensuring that all user rows are included, even if they have no matching group.
• The query then selects the desired columns: user_id, user_name, user_group_id, and group_name.

Alternative Solution

If the user prefers, they could also use the JSON_TABLE function to extract the array of IDs from the JSON field and create a virtual table. This approach allows for more flexibility in manipulating the data.

Code

<code class="sql">SELECT 
       u.user_id, 
       u.user_name, 
       uv.user_group_id
       (
           SELECT 
               g.group_name
           FROM user_group g
           WHERE g.user_group_id = uv.user_group_id
       ) AS group_name
   FROM user u
   CROSS JOIN JSON_TABLE(
       u.user_groups,
       '$[*]' COLUMNS (user_group_id INT PATH '$')
   ) uv</code>

Explanation

• The JSON_TABLE function creates a virtual table uv from the user_groups JSON field.
• The CROSS JOIN clause ensures that each row in the user table is associated with all rows in the virtual table.
• The nested subquery retrieves the group_name for each user_group_id in the uv virtual table.

By leveraging these techniques, users can effectively utilize JSON fields for joining tables and performing complex data operations in MySQL.

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