>SQL [...] is a database language [...] used for access to pseudo-relational databases that are managed by pseudo-relational database management systems (RDBMS).
>SQL is based on, but is not a strict implementation of, the relational model of data, making SQL “pseudo-relational” instead of truly relational.
>The relational model requires that every relation have no duplicate rows. SQL does not enforce this requirement.
>The relational model does not specify or recognize any sort of flag or other marker that represents unspecified, unknown, or otherwise missing data values. Consequently, the relational model depends only on two-valued (true/false) logic. SQL provides a “null value” that serves this purpose. In support of null values, SQL also depends on three-valued (true/false/unknown) logic.
Or, in other words, "relation" does not mean the relations between the tables as many assume: the tables, as a set of tuples, are the relations.
>SQL [...] is a database language [...] used for access to pseudo-relational databases that are managed by pseudo-relational database management systems (RDBMS).
>SQL is based on, but is not a strict implementation of, the relational model of data, making SQL “pseudo-relational” instead of truly relational.
>The relational model requires that every relation have no duplicate rows. SQL does not enforce this requirement.
>The relational model does not specify or recognize any sort of flag or other marker that represents unspecified, unknown, or otherwise missing data values. Consequently, the relational model depends only on two-valued (true/false) logic. SQL provides a “null value” that serves this purpose. In support of null values, SQL also depends on three-valued (true/false/unknown) logic.
Or, in other words, "relation" does not mean the relations between the tables as many assume: the tables, as a set of tuples, are the relations.