Direct product of two tables

When is direct product of two tables useful? Seems like an 开发者_StackOverflow社区academic thing.

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Check out What are the uses for cross join?

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There are some uses for it. Let's say there is a clothing store selling T-Shirts in different colours and different sizes. Each combo has it's own SKU.

If there is a table Products that references tables Sizes and Colors then

You could get list of all possible products with:

SELECT * FROM Products, Colors, Sizes

Okay that is actually quite academic.

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Some times you want to get every possible combination of rows that match certain criteria. The way to do it is cross-join and then filter out rows that do not match the criteria.

Inner/Outer joins are just a special case of this: the desired criteria is "the data of table 1 is related to the data in table2..."