Relations between tables

Question

Hello,

I have 3 tables: A, B and C. Table A is in relation (n:1) with B and with C. Typically I store in A the B.Id (or the C.Id) and the table name.

e.g.

A.ParentId = 1 A.TableName = "B"

A.ParentId = 1 A.TableName = "C"

A.ParentId = 2 A.TableName = "B"

Is it a good solution? Are there any other solutions?

Thanks in advance

Answer 1

Why not 2 parentid columns?

A.ParentIdB = 1
A.ParentIdC = 3

Answer 2

Another possibility is to introduce another table Content (D) that serves as a "supertype" to Posts and Images. Then a row in Comments (A) would reference a primary key in Content as would each row in Posts (B) and Images (D). Any common fields in Posts and Images would be moved to Content (perhaps "title" or "date") and those original tables would then only contain information specific to a post or image (perhaps "body" or "resolution"). This would make it easier to perform joins than having the table names in a field, but it does mean that a real-world entity could be both a post and a comment (or indeed, be multiply a post or comment!). Really, though, it depends on the situation that you're trying to model.