How to Find Students Belonging to Multiple Clubs Using SQL's Has-Many-Through Relationship?

Efficiently Identifying Students in Multiple Clubs using SQL's Has-Many-Through Relationship

This guide demonstrates how to retrieve students belonging to specific clubs using SQL's has-many-through relationship. We'll assume three tables: student, club, and student_club (the join table). The goal is to find students who are members of both the soccer club (ID 30) and the baseball club (ID 50).

The solution employs a concise and efficient SQL query:

<code class="language-sql">SELECT DISTINCT s.id, s.name
FROM student s
JOIN student_club sc ON s.id = sc.student_id
WHERE sc.club_id = 30
  AND s.id IN (SELECT student_id FROM student_club WHERE club_id = 50);</code>

This query leverages a subquery for optimal performance. Let's break it down:

1. JOIN Clause: The student and student_club tables are joined using the student_id to link students to their club memberships.

2. WHERE Clause: This clause filters the results. The first condition (sc.club_id = 30) ensures we only consider students in the soccer club. The second condition (s.id IN (...)) uses a subquery to further refine the results, including only students whose IDs are also present in the student_club table for the baseball club (club_id = 50).

This approach avoids unnecessary complexity and efficiently utilizes database indexes (if available on student_id and club_id columns) to quickly retrieve the desired results, even with extensive datasets. The DISTINCT keyword ensures that each student is listed only once, even if they have multiple entries in the student_club table.

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