How do I join on a function in MS Access? - sql

Question

I am trying to add a function to my JOIN condition in an MS Access query.
As a simplified example, consider that I have:
tblChildand tblParent and tblGrandparent
tblChild has both a nullable ParentId and GrandparentId depending on which one is known. I'm trying to write a query to take the more specific relationship provided. If this were TSQL it would be:
SELECT
columns
FROM
tblChild AS c
LEFT OUTER JOIN
tblParent AS p
ON p.[Id] = c.[ParentId]
LEFT OUTER JOIN
tblGrandparent AS gp
ON gp.[Id] = COALESE(p.[GrandParentId], c.[GrandParentId])
but I can't figure out how to get a COALESCE style function into the JOIN condition in Access. Here is what I would have expected to work:
SELECT
columns
FROM
(tblChild AS c
LEFT OUTER JOIN
tblParent AS p
ON p.[Id] = c.[ParentId])
LEFT OUTER JOIN
tblGrandparent AS gp
ON gp.[Id] = Nz(p.[GrandParentId], c.[GrandParentId])

Answer 1

I would expect NZ() to work. But can you use or instead?
SELECT columns
FROM (tblChild AS c LEFT JOIN
tblParent AS p
ON p.[Id] = c.[ParentId]
) LEFT JOIN
tblGrandparent AS gp
ON (gp.[Id] = p.[ParentId] or gp.[Id] = c.[GrandParentId]);
This isn't exactly the same. Formally, the last condition would be:
ON (gp.[Id] = p.[ParentId] or gp.[Id] = c.[GrandParentId] and gp.[Id] <> p.[ParentId]);
EDIT:
Another approach is two joins:
SELECT columns
FROM ((tblChild AS c LEFT JOIN
tblParent AS p
ON p.[Id] = c.[ParentId]
) LEFT JOIN
tblGrandparent AS gp
ON (gp.[Id] = p.[ParentId])
) LEFT JOIN
tblGrandparent AS gp2
ON gp.[Id] = c.[GrandParentId]);
You then have to adjust the "columns" to pull from either gp or gp2 as appropriate.