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The entity–relationship model proposes a technique that produces entity–relationship diagrams (ERDs), which can be employed to capture information about data model entity types, relationships and cardinality. A Crow's foot shows a one-to-many relationship. Alternatively a single line represents a one-to-one relationship. [4]
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 ...
An associative entity is a term used in relational and entity–relationship theory. A relational database requires the implementation of a base relation (or base table) to resolve many-to-many relationships. A base relation representing this kind of entity is called, informally, an associative table. An associative entity (using Chen notation)
Draw many simple diagrams instead. The same class (table) can appear several times on the same diagram. Use the following standard symbols to indicate the type of the relationship. One-to-one: a simple line. One-to-many, two-way: a line with a "paw". One-to-many, one-way: an arrow. Many-to-many: a line with two "paws".
Two related entities shown using Crow's Foot notation. In this example, an optional relationship is shown between Artist and Song; the symbol composed of branching lines, closest to the song entity represents "zero, one, or many", whereas a song has "one and only one" Artist, emphasized by the symbol composed of parallel lines.
The relational model (RM) is an approach to managing data using a structure and language consistent with first-order predicate logic, first described in 1969 by English computer scientist Edgar F. Codd, [1] [2] where all data is represented in terms of tuples, grouped into relations.
A relational database contains multiple tables, each similar to the one in the "flat" database model. One of the strengths of the relational model is that, in principle, any value occurring in two different records (belonging to the same table or to different tables), implies a relationship among those two records.
The model is named after database pioneer Charles Bachman, and mostly used in computer software design. In a relational model, a relation is the cohesion of attributes that are fully and not transitive functional dependent [clarify] of every key in that relation. The coupling between the relations is based on accordant attributes.