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Non-Equijoins

Equijoins are the most common type of join. They use equality comparisons (=) to join tables. Figure [*] shows our first non-equijoins.

  
Figure: Non-equijoins
\begin{figure}\begin{list}{}{
\setlength{\rightmargin}{\leftmargin}
\raggedrigh...
...~~~~~~~~p.cost~>~p2.cost
\par test->~ORDER~BY~p2.cost\end{list}\par
\end{figure}

The first query is a non-equijoin because it uses a not-equal (<>) comparison to perform the join. It returns all customers not in the same country as customernumber 648. The second query uses less-than (<) to perform the join. Instead of finding equal values to join, all rows greater-than the column's value are joined. The query returns the all employees hired after employee number 24. The third query uses greater-than (>) in a similar way. The query returns all parts that cost less than part number 153. Non-equijoins are not used very often, but there are certain queries than can only be performed using non-equijoins.


Bruce Momjian
1999-11-21