A: The number is M*N. And if we put these M*N elements in a set there will not be duplicate elements to worry about . Q: If table A have 9 columns and table B have 5 columns. What's the cartesian product of the two tables? A: There will be 9*5=45 columns. Eg: Cartesian ...
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: 结果如下:Cartesianproduct: x Assumethatattributesoftwo tables are disjoint不相交的 If attributesofr...(投影): ∏注:Duplicate rows removedfromresult, since relations aresets集合举个例子: 用投影下表: 投影结果如下(要去 mysql, LEFT OUTER JOIN 与 LEFT JOIN,RIGHT OUTER JOIN 与RIGHT JOIN ,FULL ...
When accessing a field of an ORM instance with multiple levels of inheritance and a polymorphic identity, the resulting SELECT statement has a cartesian product, even though this could be avoided using the inheritance's foreign key constraints. As far as I can tell, this only happens if the c...