Understanding the One-to-Many Relationship between Student and Course Tables

In relational databases, a one-to-many relationship exists when one record in the first table can be associated with multiple records in the second table. In this case, we have two tables: STUDENT and COURSE.

Table Structure

To understand how these tables interact, let’s take a look at their structure:

STUDENT TABLE

COURSE TABLE

As we can see, the COURSE table has a foreign key (student) that references the ID column in the STUDENT table. This establishes the one-to-many relationship between students and courses.

The Challenge

The original query attempts to join these two tables and apply filters using conditions like NOT EQUAL and NOT CONTAINS. However, when dealing with one-to-many relationships, things get complicated. In this case, we need to find all students who are not associated with the course ‘y’. But if a student is associated with multiple courses (like student ‘a’ in our example), they should only be included in the results once.

The Problem with Direct Left Joins

The original query uses a direct left join between the STUDENT and COURSE tables, which can lead to unexpected results when dealing with one-to-many relationships. By using JOIN, we’re essentially merging all records from both tables into our result set. However, when we apply filters like WHERE cour.name != 'y', we need to consider the cases where a student might appear multiple times in the results due to their association with multiple courses.

The Correct Approach: Using GROUP BY and HAVING

To solve this problem, we can use a combination of GROUP BY and HAVING clauses. Here’s an updated query that takes into account the one-to-many relationship between students and courses:

SELECT s.*
FROM student s
WHERE s.id NOT IN (
  SELECT c.student
  FROM course c
  WHERE c.name = 'y'
)

This query works by first selecting all records from the STUDENT table. Then, it uses a subquery to select the IDs of students who are associated with the course ‘y’. Finally, it filters out any student who appears in this list, effectively excluding them from our results.

Why This Works

When we use NOT IN or NOT EXISTS, PostgreSQL will return all rows that do not match the condition. In this case, we’re checking if a student’s ID is present in the list of students associated with course ‘y’. If it is, then they should be excluded from our results.

Alternative Solution: Using GROUP BY and HAVING

Another way to solve this problem is by using GROUP BY and HAVING. Here’s an example:

SELECT s.*
FROM student s
GROUP BY s.id
HAVING COUNT(DISTINCT c.name) = (
  SELECT COUNT(*)
  FROM course c
  WHERE c.student = s.id AND c.name != 'y'
)

This query works by grouping all records from the STUDENT table by their ID. Then, it uses a subquery to count the number of distinct names in the COURSE table where the student’s ID matches and the course name is not ‘y’. If this count is equal to 1, then the student should only appear once in our results.

Conclusion

In conclusion, when dealing with one-to-many relationships between tables, we need to be careful when applying filters like NOT EQUAL or NOT CONTAINS. By using techniques like grouping and having, we can effectively exclude students who are associated with multiple courses from our results. In this case, we’ve shown two alternative solutions that achieve the same goal.

Last modified on 2024-01-26