Editing Relational model (section)

== Application to relational databases ==
A '''[[data domain|data type]]''' in a relational database might be the set of integers, the set of character strings, the set of dates, etc. The relational model does not dictate what types are to be supported.

'''Attributes''' are commonly represented as '''[[Column (database)|columns]]''', '''tuples''' as '''[[row (database)|rows]]''', and '''relations''' as '''tables'''. A table is specified as a list of column definitions, each of which specifies a unique column name and the type of the values that are permitted for that column.  An '''attribute ''value''''' is the entry in a specific column and row.

A database '''[[relvar]]''' (relation variable) is commonly known as a '''base table'''. The heading of its assigned value at any time is as specified in the table declaration and its body is that most recently assigned to it by an '''update operator''' (typically, INSERT, UPDATE, or DELETE). The heading and body of the table resulting from evaluating a query are determined by the definitions of the operators used in that query.

=== SQL and the relational model ===
SQL, initially pushed as the [[standardization|standard]] language for [[relational database]]s, deviates from the relational model in several places. The current [[International Organization for Standardization|ISO]] SQL standard doesn't mention the relational model or use relational terms or concepts.{{citation needed|date=December 2023}}

According to the relational model, a Relation's attributes and tuples are [[Set (mathematics)|mathematical sets]], meaning they are unordered and unique. In a SQL table, neither rows nor columns are proper sets. A table may contain both duplicate rows and duplicate columns, and a table's columns are explicitly ordered. SQL uses a [[Null (SQL)|Null]] value to indicate missing data, which has no analog in the relational model. Because a row can represent unknown information, SQL does not adhere to the relational model's ''Information Principle''.<ref name="professionals">{{cite book |last1=Date |first1=Chris J. |title=Relational Theory for Computer Professionals: What Relational Databases are Really All About |date=2013 |publisher=O'Reilly Media |location=Sebastopol, Calif |isbn=978-1-449-36943-9 |edition=1.}}</ref>{{rp|153–155,162}}