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Liz Laffitte
23,691 PointsWhy doesn't DISTINCT always behave the way you expect in SQL queries?
Why doesn't DISTINCT always work like expected? This was my answer to the Elective Teachers problem in SQL Reporting by Example:
-- Which teachers teach elective subjects (subjects without grade levels)?
SELECT DISTINCT TEACHERS.ID, FIRST_NAME, LAST_NAME, SUBJECTS.NAME FROM TEACHERS
JOIN CLASSES ON CLASSES.TEACHER_ID = TEACHERS.ID
JOIN SUBJECTS ON SUBJECTS.ID = CLASSES.SUBJECT_ID
WHERE GRADE IS NULL;
This was Ben's Solution:
-- Which teachers teach elective subjects (subjects without grade levels)?
SELECT DISTINCT TEACHERS.ID, FIRST_NAME, LAST_NAME FROM TEACHERS
JOIN CLASSES ON CLASSES.TEACHER_ID = TEACHERS.ID
JOIN SUBJECTS ON SUBJECTS.ID = CLASSES.SUBJECT_ID
WHERE GRADE IS NULL;
Ben's solution returns one row per elective teacher. Mine returns Janis Ambrose on two lines, one for each elective she teaches. Why didn't DISTINCT return the same results when I included SUBJECTS.NAME?
2 Answers
Steven Parker
231,269 PointsThe DISTINCT restriction applies to the total result set, so if you add an additional column that makes two rows different, then both will be considered "distinct" and pass through.
Liz Laffitte
23,691 PointsThank you!