Search GROUP_CONCAT column

Suppose you produce a column using GROUP_CONCAT() that contains a bunch of IDs and you want to search that column to find rows where the column contains a particular ID. You can do so using FIND_IN_SET():

SELECT
  user_id,
  GROUP_CONCAT(project_id ORDER BY project_id) AS project_ids
FROM
  user_projects
GROUP BY
  user_id
HAVING
  FIND_IN_SET('42', project_ids) > 0;

The above query will filter the user_projects rows so that we fetch only the rows where the project_ids column which will contain something like 101, 202, 303 contains the value 42.