10. MySQL Part 1: Install, Create, Insert and SELECT

10.9 JOINs

By Ravindra Bagale · Last updated: 27 September 2026

Data vegalya tables madhe aahe – student che naav students madhe, course che naav courses madhe, score enrollments madhe. JOIN te common column var jodto.

  INNER JOIN   only matching rows in both tables
  LEFT JOIN    all rows from the left table + matches (NULL if none)
  RIGHT JOIN   all rows from the right table + matches
  CROSS JOIN   every row with every row (rarely needed)
  SELF JOIN    a table joined with itself

-- INNER: who is enrolled in what, with score
SELECT s.name, c.course_name, e.score
FROM enrollments e
JOIN students s ON s.student_id = e.student_id
JOIN courses  c ON c.course_id  = e.course_id
ORDER BY s.name;

-- LEFT: all students, even those in no course (Raja shows NULL)
SELECT s.name, e.course_id
FROM students s
LEFT JOIN enrollments e ON e.student_id = s.student_id;

-- students who have NO enrollment
SELECT s.name FROM students s
LEFT JOIN enrollments e ON e.student_id = s.student_id
WHERE e.enroll_id IS NULL;

-- RIGHT: all courses, even with no students
SELECT c.course_name, e.student_id
FROM enrollments e
RIGHT JOIN courses c ON c.course_id = e.course_id;

-- SELF JOIN: pairs of students from the same city
SELECT a.name, b.name, a.city
FROM students a JOIN students b ON a.city = b.city AND a.student_id < b.student_id;

MySQL has no FULL OUTER JOIN; combine a LEFT and a RIGHT JOIN with UNION when you need one.