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One-to-many (data model)
In systems analysis, a one-to-many relationship is a type of cardinality that refers to the relationship between two entities (see also entity–relationship model) A and B in which element of A may be linked to many elements of B, but a member of B is linked to only one element of A. For instance, think of A as mothers, and B as children. A mother can have several children, but a child can have only one mother.
In a relational database, a one-to-many relationship exists when one row in table A may be linked with many rows in table B, but one row in table B is linked to only one row in table A. It is important to note that a one-to-many relationship is not a property of the data, but rather of the relationship itself. A list of authors and their books may happen to describe books with only one author, in which case one row of the books table will refer to only one row of the authors table, but the relationship itself is not one-to-many, because books ''may'' have more than one author, forming a many-to-many relationship.
== See also ==
* One-to-one (data model)
* Many-to-many (data model)
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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