CS60 ThetaJoins by Adam Olsen.doc

CS60 ThetaJoins by Adam Olsen.doc - Theta-Joins Taken From...

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Theta-Joins Taken From: Technically, all joins are theta-joins, but by custom, if the comparison operator is equality, the join is always referred to as an equi-join or just as a join. A join based on any other comparison operator (<>, >, >=, <, <=) is a theta-join . Theta-joins are extremely rare in practice, but they can be handy in solving certain kinds of problems. These problems mostly involve finding records that have a value greater than an average or total, or records that fall within a certain range. Let's say, for example, that you've created two views, one containing the average number of units sold for each product category and a second containing the total units sold by product, as shown in Figure 5-6. We'll look at how to create these views later in this chapter. For now, just assume their existence. Figure 5-6: These views can be joined using a theta-join.
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The following SELECT statement, based on the comparison operator >, will produce a
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