how to GROUP BY so that not all results are grouped

When I use my query to select all data from the table I have a problem with the custnumber field. If custnumber is returned within the same month then it should group the results but only then. This is my query: SELECT MONTH(periode) AS maand, YEAR(periode) AS jaar, maandfacturen.verzenddatum, maandfacturen.factuurnr, maandfacturen.custnumber, klantgegevens.bedrijfsnaam AS bedrijfsnaam, maandfacturen.totaalbedragexcl, maandfacturen.totaalbedragincl, maandfacturen.betaald FROM maandfacturen LEFT JOIN klantgegevens ON maandfacturen.custnumber = klantgegevens.custnumber WHERE periode LIKE '2023%' GROUP BY custnumber ORDER BY periode ASC, factuurnr ASC This query does not work cause if I use GROUP BY custnumber it groups all records and thats not the result I need as all months also get grouped. After googling I found that I might need to use CASE to solve this but I am not sure how to use this. Any help would be greatly appreciated.