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Relation, tuple, and attribute represented as table, row, and column respectively. In database theory, a relation, as originally defined by E. F. Codd, [1] is a set of tuples (d 1,d 2,...,d n), where each element d j is a member of D j, a data domain. Codd's original definition notwithstanding, and contrary to the usual definition in ...
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)
Relational databases store tabular data represented as relations. Queries over relational databases often likewise return tabular data represented as relations. The main purpose of relational algebra is to define operators that transform one or more input relations to an output relation. Given that these operators accept relations as input and ...
A table in a SQL database schema corresponds to a predicate variable; the contents of a table to a relation; key constraints, other constraints, and SQL queries correspond to predicates. However, SQL databases deviate from the relational model in many details , and Codd fiercely argued against deviations that compromise the original principles.
For example, consider a database of electronic health records. Such a database could contain tables like the following: A doctor table with information about physicians. A patient table for medical subjects undergoing treatment. An appointment table with an entry for each hospital visit. Natural relationships exist between these entities:
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.
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So for example, would mean () since it would be associated with the logical statement = and similarly, would mean () since it would be associated with = (). Sometimes, set complement (subtraction) ∖ {\displaystyle \,\setminus \,} is also associated with logical complement (not) ¬ , {\displaystyle \,\lnot ,\,} in which case it will have the ...