Basics: query specification

re: Basics: query specification

Martin — Fri, 24 Sep 2004 20:12:00 GMT

In my experience the union and union all are some of the most counterintuitive in all of SQL. For the company I work at we give an interview test to candidates. As part of the test the candidates are required to write a query using the union statement. Almost invariably the answers apply the select distinct on the a subquery containing the union to eliminate duplicates. Often the answers include comments talking about how important this is to eliminate duplicates. the UNION statement implicitly eliminates all duplicates. If you want duplicates then you must use the UNION ALL statement.
The reason for this is as I understand it is that the UNION statement is a set operator (remember your courses in statistics) borrowed from mathematics and really is outside of traditional programming language concepts which are based on coding algorithms.

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