The semantics is described by means of an extended relational algebra with new operators: Group by, Unnest
, Extend, Substitute, Rename, Powerset, which transform a relational table into an object-relational table (i.
Monoid comprehension calculus treats operations over multiple collection types, aggregates, and quantifiers in a similar way, resulting in a uniform method to unnest queries, regardless of their type of nesting.
f  [bar]s}(u), [bar]q} are very difficult to normalize and unnest.
unnest, [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], returns the collection of all pairs (x, y) for each x [element of] X and for each y [element of] x.
Note that Rule (C2) compiles every comprehension generator into an unnest.
The semantics of these operators cannot be given in terms of the monoid calculus if the outer query constructs a nonidempotent collection, since information about the exact number of copies of the data in the outer query may be lost after the outer join or unnest.
Our algorithm requires only two rewrite rules to unnest the queries that cannot be handled by the normalization algorithm.
The rules in Figure 10 unnest all nested comprehensions.
Even though Rule (C11) has a precondition, it will eventually be applied to unnest any nested query in a predicate.
Second, in contrast to joins, there are very few ways to evaluate the unnest operator.
La semantica se describe por medio de una extension del algebra relacional con nuevos operadores: Group by (G), Unnest
(h), Extended (E), Substitute (a), Rename (r), Powerset (R), que permiten transformar una relacion con el fin de descubrir reglas de asociacion  .