SELECT * FROM result;
SELECT DISTINCT * FROM result, grade
WHERE result.grade = grade.score;
- The cardinality of the resultant relation instance is equal to that of the result table.
- If we project the resultant table on scale we do not get all the scale rows.
For each row in the result table find the row in the grade table such that the result's grade value is equal to that of the grade's scale value.
SELECT DISTINCT *
FROM result, grade;
This is an expensive operation!
Each row of the result table is concatenated with each row of the grade table.