Recently I was working on a D7 to D8 migration. I was trying to import news items and their taxonomy terms (among many other things). To make things simple I wanted the query results to have (for each node) a single field that contained a comma-separated list of taxonomy terms which I could then explode during the row processing. Using GROUP_CONCAT we can achieve this! Let’s break it down:

The Drupal 7 site has the following structure (focusing on the important bits for this blog post):

The migration relies on the d7_node migrate source plugin, which basically queries for nodes of a specific type. The query object looks like this (simplified for this blog post):