Conceptual Evaluation
The cross-product of relation-list is computed, tuples that fail qualification are discarded, `unnecessary’ fields are deleted, and the remaining tuples are partitioned into groups by the value of attributes in grouping-list.
The group-qualification is then applied to eliminate some groups. Expressions in group-qualification must have a single value per group!
- In effect, an attribute in group-qualification that is not an argument of an aggregate op also appears in grouping-list. (SQL does not exploit primary key semantics here!)
One answer tuple is generated per qualifying group.