Referring to the join conditions explained here Tables connection not reflect all data - #4 by darcoli
Since you have a Left join with a join condition matching order_id on both tables, it seems that the dealcodes table has some order_id’s that are not present in the revenue_refunds table. If this is not expected then you need to confirm that the data in the datasource is consistent - double check this with the owner/administrator of your datasource tables. Otherwise choose a different join condition (inner, left, right or outer) that makes for your use case.