<P> "Table" is another term for "relation"; although there is the difference in that a table is usually a multiset (bag) of rows where a relation is a set and does not allow duplicates . Besides the actual data rows, tables generally have associated with them some metadata, such as constraints on the table or on the values within particular columns . </P> <P> The data in a table does not have to be physically stored in the database . Views also function as relational tables, but their data are calculated at query time . External tables (in Informix or Oracle, for example) can also be thought of as views . </P> <P> In terms of the relational model of databases, a table can be considered a convenient representation of a relation, but the two are not strictly equivalent . For instance, an SQL table can potentially contain duplicate rows, whereas a true relation cannot contain duplicate tuples . Similarly, representation as a table implies a particular ordering to the rows and columns, whereas a relation is explicitly unordered . However, the database system does not guarantee any ordering of the rows unless an ORDER BY clause is specified in the SELECT statement that queries the table . </P> <P> An equally valid representation of a relation is as an n - dimensional chart, where n is the number of attributes (a table's columns). For example, a relation with two attributes and three values can be represented as a table with two columns and three rows, or as a two - dimensional graph with three points . The table and graph representations are only equivalent if the ordering of rows is not significant, and the table has no duplicate rows . </P>

If a table contains a primary key there can be no duplicate records in a table