SQL Query with Joins

Question

Hi..I'm having trouble wiht my query.
I'm getting way too many rows when displayed.
I should have either 12 or 24 row...I think 12, but not sure.
What I am getting is 228 rows displayed.
This is all my code, including the select statment that dsiplays rows.
--Begin Create View Casses_Schedule(2)
Drop view classes_schedule;
Create View Classes_Schedule
AS SELECT
Class_ID,
School_year,
Semester,
Course.Course_ID,
Class_Location.Class_Room,
Class_Location.Class_Building,
Title,
Starting_time,
Day,
Duration
FROM
Class, Course,
Class_Location,
Schedule_Type_Details
WHERE
semester ='SPRING'
AND
School_Year= To_Char(To_Date('1/mar/1997'))
AND
Class.Course_ID = Course.Course_ID
AND
Class.Schedule_ID = Schedule_Type_details.Schedule_ID
AND
Class.Class_Building = Class_Location.Class_Building
ORDER BY
Course.Title, Class.Class_ID;
--Query to display view data
SELECT
SubStr(Class_ID,1,7) "Class",
SubStr(School_year,1,10) "Year",
SubStr(Semester,1,10) "Semester",
SubStr(Class_Room,1,4) "Loc",
SubStr(Class_Building,1,15) "Bldng",
SubStr(Title,1,20) "title",
SubStr(Starting_time,1,6) "STime",
SubStr(Day,1,2)"Day",
SubStr(Duration,1,3) "Period"
FROM
Classes_Schedule;
Thankyou.

Answer 1

Suggestions:
You can use "CREATE OR REPLACE VIEW" instead of "DROP VIEW" and "CREATE VIEW", although either will work.
Preface column names with table names to avoid ambiguity.
Select from the fewest tables possible. In other words, if some columns are in multiple tables, and you can get all the columns you want from one of those tables, then don't use the others; It just overly complicates things. It looks like you can get all the columns you want from the class, course, and schedule_type_details tables, so eliminate the class_location and schedule_type tables. For example, instead of joining class.schedule_id to schedule_type.schedule_id and joining schedule_type.schedule_id to schedule_type_details.schedule_id, just join class.schedule_id to schedule_type_details.schedule_id.
Make sure you have all the join conditions that you need. For example, is the course_id unique by itself? Or, does it require a combination of course_id and department_id to be unique? You may need to add a join condition for department_id. If this is what you are missing, then this
would account for having more rows than expected.
What data type is school_year? Is it VARCHAR2 or NUMBER or DATE? You have used TO_DATE to convert the VARCHAR2 string '1/mar/1997' to a DATE, then used TO_CHAR to convert it back to VARCHAR2, which seems pointless. If school_year is a DATE, then you want to convert '1/mar/1997' to a DATE for comparison. The best way to do that is to use TO_DATE and supply a date format,
to avoid any problems with implicit conversions and conflicting date formats.
Please see suggested corrected code below:
CREATE OR REPLACE VIEW classes_schedule
AS
SELECT   class.class_id,
         class.school_year,
         class.semester,
         class.course_id,
         class.class_room,
         class.class_building,
         course.title,
         schedule_type_details.starting_time,
         schedule_type_details.day,
         schedule_type_details.duration
FROM     class,
         course,
         schedule_type_details
WHERE    class.course_id = course.course_id
AND      class.department_id = course.department_id
AND      class.schedule_id = schedule_type_details.schedule_id
AND      class.semester = 'SPRING'
AND      class.school_year = TO_DATE ('1/mar/1997', 'dd/mon/yyyy')
ORDER BY course.title,
         class.class_id