Oracle 11g Release 2

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CREATE TABLE CLASSES (CLASS_ID, TEACHER) AS
  SELECT 61, 'Mrs. Green' FROM DUAL UNION ALL
  SELECT 62, 'Mr. Brown' FROM DUAL UNION ALL
  SELECT 63, 'Mr. Black' FROM DUAL UNION ALL
  SELECT 64, 'Ms. Tan' FROM DUAL UNION ALL
  SELECT 65, 'Mr. Miller' FROM DUAL UNION ALL
  SELECT 66, 'Mrs. Jones' FROM DUAL UNION ALL
  SELECT 80, 'Mrs. Brown' FROM DUAL UNION ALL
  SELECT 81, 'Ms. Wan' FROM DUAL UNION ALL
  SELECT 92, 'Mr. Bland' FROM DUAL;
9 rows affected

CREATE TABLE TRIPS (CLASS_ID, TRIP) AS
  SELECT 62, 'Zoo' FROM DUAL UNION ALL
  SELECT 62, 'Park' FROM DUAL UNION ALL
  SELECT 62, 'Observatory' FROM DUAL UNION ALL
  SELECT 64, 'Park' FROM DUAL UNION ALL
  SELECT 64, 'Zoo' FROM DUAL UNION ALL
  SELECT 81, 'Park' FROM DUAL;
6 rows affected

WITH cteTotal_class_count AS (SELECT COUNT(*) AS TOTAL_CLASS_COUNT
                                    FROM CLASSES),
         cteObservatory_class_count AS (SELECT COUNT(DISTINCT CLASS_ID) AS OBSERVATORY_CLASS_COUNT
                                          FROM TRIPS
                                          WHERE UPPER(TRIP) = 'OBSERVATORY')
    SELECT TOTAL_CLASS_COUNT - OBSERVATORY_CLASS_COUNT AS COUNT_OF_CLASSES_WHO_DIDNT_GO
      FROM cteTotal_class_count
      CROSS JOIN cteObservatory_class_count


COUNT_OF_CLASSES_WHO_DIDNT_GO


8