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One-to-many (data model)
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<p>In <a href="/facts/Systems_analysis/2NwqATNQ">systems analysis</a>, a one-to-many relationship is a type of <a href="/facts/Cardinality_(data_modeling)/SWaMnYbC">cardinality</a> that refers to the relationship between two entities (see also <a href="/facts/Entity%25E2%2580%2593relationship_model/U8A0mwYe">entity–relationship model</a>). For example, take a car and an owner of the car. The car can only be owned by one owner at a time or not owned at all, and an owner could own zero, one, or multiple cars. One owner could have many cars, one-to-many. </p><p>In a <a href="/facts/Relational_database/pEKGmJLy">relational database</a>, a one-to-many relationship exists when one record is related to many records of another table. A one-to-many relationship is not a property of the data, but rather of the relationship itself. One-to-many often refer to a primary key to foreign key relationship between two tables, where the record in the first table can relate to multiple records in the second table. A foreign key is one side of the relationship that shows a row or multiple rows, with one of those rows being the primary key already listed on the first table. This is also called a <a href="/facts/Foreign_key/52Ns8RTD">foreign key constraint</a>, which is important to keep data from being duplicated and have relationships within the database stay reliable as more information is added. </p><p>Many-to-many relationships are not able to be used in relational databases and must be converted to one-to-many relationships. Both one-to-many and one-to-one relationships are common in relational databases. </p><p>The opposite of one-to-many is many-to-one. The <a href="/facts/Converse_relation/EVGlRSz7">transpose</a> of a one-to-many relationship is a many-to-one relationship. </p>