Question

Let’s assume I have a table called normalize with an enum column called type

Depending on the row (Let’s assume the types are a, b or c) I want to do a LEFT JOIN based on whatever the type is.

So, if type = a, LEFT JOIN ON table a .. if type = B, LEFT JOIN ON table b… etc

Can this be done efficiently? Or should this be done at the application layer?

Answer 1

Yes, you should be able to do something like this:

SELECT *
FROM parent
    LEFT JOIN a ON (type = 'a' AND parent.id = a.id)
    LEFT JOIN b ON (type = 'b' AND parent.id = b.id)
    LEFT JOIN c ON (type = 'c' AND parent.id = c.id)

You may or may not need an index on type, depending on the actual data and how smart your DBMS is (in exploiting indexes that ought to exist underneath PKs). In any case, measure on realistic amounts of data.