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In the interest of completeness, and to provide a tool that is extremely useful in dealing with heterogeneous data, it’s time we take a look at the UNION operator. SQL and setsIf you were to dredge up the memories of middle school, you’ll probably remember vague notions of learning about sets. If you have ever studied logic, you may have even worked with Venn diagrams. Sets are simply groups of discrete elements. In the strict mathematical world, each set is made up of unique numeri 
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