Question

If I use null as a representation of everything in a database table is that bad practice ?

i.e.

I have the tables: myTable(ID) and myRelatedTable(ID,myTableID)

myRelatedTable.myTableID is a FK of myTable.ID

What I want to accomplish is: if myRelatedTable.myTableID is null then my business logic will interpret that as being linked to all myTable rows.

The reason I want to do this is because I have an uknown amount of rows that could be inserted into myTable after the myRelatedTable row is created and some rows in the myRelatedTable need to reference all existing rows in myTable.

Answer 1

I don’t think NULL is the best way to do it but you might use a separate tinyInt column to indicate that the row in MyRelatedTable is related to everything in MyTable, e.g. MyRelatedTable.RelatedAll. That would make it more explicit for other that have to maintain it. Then you could do some sort of Union query e.g.

SELECT M.ID, R.ID AS RelatedTableID,.... FROM MyTable M INNER JOIN MyRelated Table R ON R.myTableId = M.Id UNION SELECT M.ID, R.ID AS RelatedTableID,.... FROM MyTable M, MyRelatedTable R WHERE R.RelatedAll = 1