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Relationships
General
Relationship is a link
between a pair of tables. It exists if two tables are linked by primary key
and foreign key
Each relationship is described by the type of relationship
which exists between two tables, the type of
participation and the degree of participation
these tables.
one-to-one relationship
It occurs when a single record from the table A is precisely linked with
only one record from table B and vice versa.
one-to-many relationship
It occurs when a single
record from the table A may
correspond to one or more records of
table B, but with a single record
table B corresponds to a maximum of one
record in table A. It is not so mutual relationship
as in the case of one-to-one.
identyfying relationships
Foreign key, which is a component of
complex primary key in the dependent relation
is defined as the primary foreign key and
so constructed tie is named as identyfing relation
many-to-many relationship
It occurs when one record from table A is linked with many records from table B and
one of the records from table B is linked with many records from table A
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