Only exposures with certain filters need to find fringe data. If the exposure under consideration is in one of those filters, the current Gen3 butler sql JOINs should find it correctly. However, for filters that do not need a fringe, the current JOINs will return an empty set, as there is no valid fringe for that filter. Doing this in pure SQL would suggest we want a LEFT JOIN to the fringe data, so we can either return the matching data or NULL, and deal with the consequences downstream, but my understanding is that we do not support that kind of JOIN.