>>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.
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>Thomas,
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>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?
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>Renoir

To add to what Tom said, another scenario is the case where you want to add a person to your system, and you don't know the SSN yet. If the SSN is the primary key, you can't add the person to the database.