How to implement superkey sub key method on cascading tables If I design a database with a securitymaster table that has all the common columns and then have a final column called securitytype which acts as a reference to the sub table, how is it possible to return the extended row (that joins the securitymaster with the sub table in a UDF or Stored Proc)?

In my example, I will abbreviate the securitymaster table as A, the subtables are B,C,...,Z. Each subtables has its own specific columns. Let's further imagine the A has the first column a1 which is the PK and the last column a10 that holds the value B,...,Z (I.e. a FK to the subtable name). The number of columns in the sub table differ and will normally be specific to each table, however some tables may share the same column names.

If I have a securityID, say @sID, then, I will need to join the table as defined in A.a10 where A.a1 = @sID. I will then need to present select * from F (let's imagine A.a10 was equal to F). I have tried with Execute, but people say its a bad method, also I don't know how to return the value or the combined row from an EXEC within a sp. UDF looks impossible as they can not be dynamic. An example would be really useful as I feel this should be a fundamental tenant of any DB?

Many thanks
Normally, you'd create a view for each subtype. Do the join between the "super" table and the "sub" table in the view. 

If you make the views updatable, client code can just use the views.