DBMS: One-to-One Relationship (Definitions)

"A single instance of one entity is associated with a single instance of another entity." (Owen Williams, "MCSE TestPrep: SQL Server 6.5 Design and Implementation", 1998)

"A relationship between two tables in which a single row in the first table can be related only to one row in the second table, and a row in the second table can be related to only one row in the first table. This type of relationship is unusual." (Microsoft Corporation, "SQL Server 7.0 System Administration Training Kit", 1999)

"A relationship type between tables where one row in a given table is related to only one or zero rows in a second table. This relationship type is often used for subtyping. For example, an EMPLOYEE table may hold the information common to all employees, while the FULLTIME, PARTTIME, and CONTRACTOR tables hold information unique to full-time employees, part-time employees, and contractors, respectively. These entities would be considered subtypes of an EMPLOYEE and maintain a one-to-one relationship with the EMPLOYEE table." (Bob Bryla, "Oracle Database Foundations", 2004)

"Occurs when one row or thing of an entity is associated with only one row or thing of another. One-to-one relationships are uncommon in the real world." (Thomas Moore, "EXAM CRAM™ 2: Designing and Implementing Databases with SQL Server 2000 Enterprise Edition", 2005)

"The relationship between two tables dictated by having one record in each table, and not more than one record in either table, related back to the other table." (Gavin Powell, "Beginning Database Design", 2006)

"A relationship between two tables in which a single row in the first table can be related to only one row in the second table, and a row in the second table can be related to only one row in the first table." (S. Sumathi & S. Esakkirajan, "Fundamentals of Relational Database Management Systems", 2007)

"Used in a relational database to denote that a single row in the parent table can be related to only one row in the related child table and that a row in the child table can be related to only a single row in the referenced parent table." (Victor Isakov et al, "MCITP Administrator: Microsoft SQL Server 2005 Optimization and Maintenance (70-444) Study Guide", 2007)

"Occurs when one record in a table corresponds to exactly one record in another table." (Rod Stephens, "Beginning Database Design Solutions", 2008)

"One of three types of relationships (associations among two or more entities) that are used by data models. In a 1:1 relationship, one entity instance is associated with only one instance of the related entity." (Carlos Coronel et al, "Database Systems: Design, Implementation, and Management" 9th Ed., 2011)

"A link between two entities in which the cardinality of both sides of the relationship is one." (Craig S Mullins, "Database Administration", 2012)

"A relationship between two entities in a database such that each instance of an entity is related to no more than one instance of the other entity." (Jan L Harrington, "Relational Database Design and Implementation" 3rd Ed., 2009)