MySQL - Group By - How does it determine which row to return from joined table

I am getting desired, but inexplicable behavior on a particular query, and wanted to get more information before pushing up. I have 3 tables: **foo** **bar** **bar_image** And paring back all distractors, the fundamental query looks like this: SELECT f.id AS foo_id, b.id AS bar_id, COALESCE(bi.image_path, b.legacy_image_path) AS image_path FROM foo AS f INNER JOIN bar AS b ON b.foo_id = f.id LEFT OUTER JOIN bar_image AS bi ON bi.bar_id = b.id AND bi.deleted_at > f.created_at GROUP BY f.id And the sample sets: ; The following query will result in image_path = a.png due to no matching rows in bar_image foo id: 1 created_at: 1000 bar id: 1 legacy_image_path: a.png bar_image {empty} ; New foo row created, after image replacement ; Above query will return ; foo: 1, image_path: a.png ; foo: 2, image_path: b.png foo id: 2 created_at: 2000 bar id: 1 legacy_image_path: b.png bar_image id: 1 image_path: a.png deleted_at: 1500 ; And another iteration for sake of thoroughness ; Another foo row created, after another image replacement ; Above query will return ; foo: 1, image_path: a.png ; foo: 2, image_path: b.png ; foo: 3, image_path: c.png foo id: 3 created_at: 3000 bar id: 1 legacy_image_path: c.png bar_image id: 2 image_path: b.png deleted_at 2500 As you can see, with the group by on the foo table, and the given join to the bar_image table, each foo is correctly displaying the proper image_path, which is desired, but I cannot explain why this is occurring. When running the query after all the new rows were inserted, foo:1 would return two image rows from bar_image table, but the group by is truncating them to a single row, how does MySQL know to select the closest value, vs some other (seemingly random) row?