November 2014 Archive


Correlated Subqueries

The simplest subqueries can be evaluated once and for all, and the result used in a higher-level query. A more complicated use of nested subqueries requires the subquery to be evaluated many times: once for each assignment of a value to some term in the subquery that comes

Subqueries in FROM Clauses

Another use for subqueries is as relations in a FROM clause. In a FROM list, instead of a stored relation, we may use a parenthesized subquery. Since we don't have a name for the result of this subquery, we must give it a tuple-variable alias.

Natural Joins / Outerjoins

As we recall from Natural Joins / Theta-Joins, a natural join differs from a theta-join in that: 1. The join condition is that all pairs of attributes from the two relations having a common name are equated, and there are no other conditions. 2. One of each pair of equated attributes is projected out.

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