How To Software The Definition of a Candidate Key Database candidate keys sometimes become primary keys Share Pin Email Print matejmo / Getty Images Software Databases Documents Spreadsheets Presentations Desktop Publishing Graphic Design Animation & Video By Mike Chapple Writer Former Lifewire writer Mike Chapple is an IT professional with more than 10 years' experience cybersecurity and extensive knowledge of SQL and database management. our editorial process Twitter Mike Chapple Updated November 04, 2019 52 52 people found this article helpful 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. 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 uniquely identify each employee a company might use the employee's Social Security number. As you can see, there are people with the same first names, last names, and position, but no two people ever have 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 have 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.) Continue Reading How to Choose a Primary Key for Your Database How to Identify a Database Record Using a Superkey Primary Keys in a Database and What Makes a Good Key What Are Some Keys Take Make DB Management Easier? How BCNF Is Used in Relational Database Theory Protect Your Data by Normalizing It to at Least 2NF A Quick Guide to Understanding Database Dependencies The Role of Referential Integrity in Your Database Database Terms You Need to Know 5 Things Beginners Need to Know About Databases Serial Numbers: Everything You Need to Know Ditch Your Spreadsheet for a Database to Access the Power of Your Data Relationships Are the Basis for Relational Databases A Database Relation Might Not Be What You Think Definition of a Determinant in a Database What Is a UNIQUE Constraint in a SQL Server?