Clear Explanation Of The "theta Join" In Relational Algebra?
Answer : Leaving SQL aside for a moment... A relational operator takes one or more relations as parameters and results in a relation. Because a relation has no attributes with duplicate names by definition, relational operations theta join and natural join will both "remove the duplicate attributes." [A big problem with posting examples in SQL to explain relation operations, as you requested, is that the result of a SQL query is not a relation because, among other sins, it can have duplicate rows and/or columns.] The relational Cartesian product operation (results in a relation) differs from set Cartesian product (results in a set of pairs). The word 'Cartesian' isn't particularly helpful here. In fact, Codd called his primitive operator 'product'. The truly relational language Tutorial D lacks a product operator and product is not a primitive operator in the relational algebra proposed by co-author of Tutorial D, Hugh Darwen**. This is because th...