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The data in a table does not have to be physically stored in the database. Views are also relational tables, but their data is calculated at query time. Another example are nicknames, which represent a pointer to a table in another database. COMPARISONS WITH OTHER DATA STRUCTURES In non-relational systems, such as Hierarchical Databases , the distant counterpart of a table is a structured File , representing the rows of a table in each record of the file and each column in a record. Unlike a Spreadsheet , the Datatype of each field is ordinarily defined by the Schema describing the table. Some relational systems are less strict about field datatype definitions. TABLES VERSUS RELATIONS 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 Tuple s. 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.An equally valid representation of a relation is as an ''n''-dimensional Graph , 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. SEE ALSO |
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