Joining Tables with Comma-Separated Values

Database normalization is crucial for efficient data management, but sometimes, projects may require handling legacy data in non-normalized form. This article explores a technique for joining two tables with comma-separated values in a given column.

Scenario:
Suppose we have two tables:

• Notes Table:

  • nid: Note ID
  • forDepts: Comma-separated department IDs

• Positions Table:

  • id: Position ID
  • name: Position name

Our goal is to join these tables and associate the forDepts column with values from the Positions table, creating a new column with comma-separated position names.

Solution Using CONCAT_WS:

SELECT n.nid,
       CONCAT_WS(', ', p.name) AS DepartmentName
FROM   Notes n
JOIN   Positions p
       ON n.forDepts = p.id

However, this approach will only retrieve the first match for each department ID, not all matching departments.

Solution Using FIND_IN_SET:

SELECT  a.nid,
        GROUP_CONCAT(b.name ORDER BY b.id) AS DepartmentName
FROM    Notes a
        INNER JOIN Positions b
            ON FIND_IN_SET(b.id, a.forDepts) > 0
GROUP   BY a.nid

The FIND_IN_SET function checks if a value is present in a comma-separated string. By using it in the JOIN condition, we ensure that all matching positions are included. The GROUP_CONCAT function concatenates the retrieved position names, separated by commas.

PHP Code Integration:

The given PHP code can be modified to retrieve the extended DepartmentName column by replacing the query in the mysql_query statement with the suggested solution above. This will allow the code to export the comma-separated position names correctly.

Note:
While it's ideal to normalize databases, handling non-normalized data is sometimes necessary. The techniques discussed in this article provide a solution for efficiently joining tables and aggregating comma-separated values in such scenarios.