Software & Apps > Apps 63 63 people found this article helpful The Definition of a Candidate Key Database candidate keys sometimes become primary keys By Mike Chapple Mike Chapple Twitter Writer University of Idaho Auburn University Notre Dame Former Lifewire writer Mike Chapple is an IT professional with more than 10 years' experience cybersecurity and extensive knowledge of SQL and database management. lifewire's editorial guidelines Updated on June 20, 2020 Tweet Share Email Apps Best Apps Payment Services A candidate key is a combination of attributes that uniquely identify a database record without referring to any other data. Each table may have one or more candidate. One of these candidate keys is selected as the table primary key. A table contains only one primary key, but it can contain several candidate keys. If a candidate key is composed of two or more columns, then it's called a composite key. enot-poloskun / Getty Images Properties of a Candidate Key All candidate keys have some common properties. One of the properties is that for the lifetime of the candidate key, the attribute used for identification must remain the same. Another is that the value cannot be null. Lastly, the candidate key must be unique. For example, to specifically and uniquely identify each employee, a company might use the employee's Social Security number. Some people share the same first names, last names, and position, but no two people use the same Social Security number. Social Security Number First Name Last Name Position 123-45-6780 Craig Jones Manager 234-56-7890 Craig Beal Associate 345-67-8900 Sandra Beal Manager 456-78-9010 Trina Jones Associate 567-89-0120 Sandra Smith Associate Examples of Candidate Keys Some types of data readily lend themselves as candidates: International Standard Book Numbers: ISBNs uniquely identify books and related media. The issuance of ISBNs is tightly regulated by industry gatekeepers and ISBNs are never re-used by publishers. Bank account numbers: Most banks do not recycle account numbers. Serial numbers: Although serial numbers aren't governed across industries, in the context of a single supplier, a serial number should always be unique. Driver license numbers: Usually, these numbers are not duplicated. However, a person who moves from state to state can have more than one DL number. National Provider ID: Physicians and other licensed medical providers each have at least one NPI that's unique to them, issued by the U.S. Department of Health and Human Services. However, some types of information that might seem like good candidates actually prove problematic: Phone numbers: Most carriers recycle phone numbers, and individual subscribers can use several phone numbers simultaneously. Universal Price Codes: UPCs are unique, but the owner of a UPC block can recycle products at will.Medical record numbers: MRNs are generally issued on a hospital level, without any sort of national guidance about the structure and format of these identifiers.Social Security Numbers: Although they're theoretically unique, SSNs do get recycled, and SSN fraud is common enough to make this identifier problematic across large data sets. (In the context of an employer that verifies SSNs, this challenge isn't a problem.) Was this page helpful? Thanks for letting us know! Get the Latest Tech News Delivered Every Day Email Address Sign up There was an error. Please try again. You're in! Thanks for signing up. There was an error. Please try again. Thank you for signing up! Tell us why! Other Not enough details Hard to understand Submit More from Lifewire Basic Keys That Make Database Management Easy What Is a Database Relationship? What Is a Primary Key? Definition of Database Relation Full Functional Dependency in Database Normalization What Is Boyce-Codd Normal Form (BCNF)? Choosing a Primary Key What Are Database Dependencies? Glossary of Common Database Terms Entity-Relationship Diagram Definition 5 Best Database Tips for Beginners What Is a Database? Why We Fall for Texting Scams (and How to Stop) Putting a Database in First Normal Form What Is a Serial Number? What Is the Definition of a Database Query?