Nate Cox

Yeah this strikes me as safeguarding against a possible bad decision.

No, who said there was a relationship?

A compound key is a composite key where one or both sides can be foreign keys to other tables themselves; it’s a safe assumption this is probably true in a large data set like social security. A composite key is a candidate key (a uniquely identified key) made up of more than one column.

This basically means that there is a finite number of available SSNs because they’re only 10 digits long and someone intends to recycle SSNs after the current user of one dies. Linking it to birthday is “unique enough” as to never recur.

Because a simple query would have shown that SSN was a compound key with another column (birth date, I think), and not the identifier he thinks it is.