Can MySQL JSON Fields Be Used for Table Joining?

Using MySQL JSON Field for Table Joining

In database management, it is often necessary to join data from multiple tables to obtain comprehensive insights. When working with MySQL, you may encounter situations where you have a JSON field storing a list of identifiers. This post addresses the question of whether it is feasible to execute operations on such JSON fields and utilize them in SQL queries.

The Problem

Consider the following fictitious scenario: You have a "user" table with columns for user ID, username, and a JSON field called "user_groups" that stores a list of group IDs. There is also a "user_group" table with columns for group ID and group name. The goal is to construct a query that retrieves information about users and their respective groups, resulting in a table with the following structure:

user_id | user_name | user_group_id | group_name

The desired results would be:

101     | John      | 1             | Group A
101     | John      | 3             | Group C

The Solution

To achieve the desired results, you can employ the following query:

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), '$')

The "JSON_CONTAINS" function is used to determine whether a user group ID exists in the JSON field "user_groups." By utilizing this function, you can join the "user" and "user_group" tables based on the presence of group IDs in the JSON field. This enables the retrieval of information about users and their associated groups.

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