XI-depth understanding of the principles and join query

Others 2019-10-30 12:57:03 views: null

When we query data from multiple tables, we need to use join queries, join queries usage is very high.

First, the Cartesian product

　　Simply understood Cartesian product: There are two sets A and B, represented by Cartesian product of the set of elements A and B set in the elements of any interconnected generation of all possible outcomes.

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Origin www.cnblogs.com/biao/p/11764055.html

XI-depth understanding of the principles and join query

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