>A primary key is unique and identifies a specific record. A candidate key is also unique, however it relates to an entity other than the primary key. An example would be a customer code, social security code, etc. If a violation of uniqueness occurs due to a duplicate – the system user must know. It has been known to have duplicate social security numbers issued. When this occurs it must be resolved. Hope you get the general concept.

Thomas,

PMFJI, but do you mean that if I have a uniquie control number for each record that could be the primary key, but if I also have a business need to make sure the social security number is unique, that would be a candidate? Silly question, I'm sure, but when do you need both? Why not just use the SSN?

Renoir