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György Varga19,198 Points
A more complex solution
Please check out my solution. I've got the same teacher back. Is it good?
WITH ONEPERIODS AS ( SELECT PERIODS.ID AS "Periods ID" FROM PERIODS LEFT JOIN CLASSES ON PERIODS.ID = CLASSES.PERIOD_ID WHERE PERIODS.ID = 1 ), FIRST_CLASS AS ( SELECT DISTINCT TEACHERS.ID AS "Teachers ID" FROM TEACHERS LEFT JOIN CLASSES ON TEACHERS.ID = CLASSES.TEACHER_ID LEFT JOIN ONEPERIODS ON CLASSES.PERIOD_ID = ONEPERIODS."Periods ID" WHERE ONEPERIODS."Periods ID" IS NOT NULL ) SELECT TEACHERS.ID, TEACHERS.FIRST_NAME, TEACHERS.LAST_NAME FROM TEACHERS LEFT JOIN FIRST_CLASS ON TEACHERS.ID = FIRST_CLASS."Teachers ID" GROUP BY TEACHERS.ID HAVING FIRST_CLASS."Teachers ID" IS NULL
It's wonderful György!