📊 Union of A and B. 3.5 CROSS JOIN Cartesian product – every row in A paired with every row in B. (Use with caution!)
SELECT * FROM students INNER JOIN courses ON students.id = courses.student_id; | id | name | student_id | course | |----|-------|------------|---------| | 1 | Alice | 1 | Math | | 2 | Bob | 2 | Science |
SELECT * FROM students LEFT JOIN courses ON students.id = courses.student_id; | id | name | student_id | course | |----|---------|------------|---------| | 1 | Alice | 1 | Math | | 2 | Bob | 2 | Science | | 3 | Charlie | NULL | NULL | Returns ALL rows from the right table, matched rows from the left table. NULL if no match. sql joins notes pdf
📊 Intersection of A and B. 3.2 LEFT JOIN (or LEFT OUTER JOIN) Returns ALL rows from the left table, matched rows from the right table. NULL if no match.
SELECT * FROM students FULL OUTER JOIN courses ON students.id = courses.student_id; | id | name | student_id | course | |------|---------|------------|---------| | 1 | Alice | 1 | Math | | 2 | Bob | 2 | Science | | 3 | Charlie | NULL | NULL | | NULL | NULL | 4 | Art | 📊 Union of A and B
SELECT e.name, d.dept_name FROM employees e JOIN departments d ON e.dept_id = d.dept_id; Assume two tables:
-- Query: Books with author names SELECT b.title, a.name FROM books b LEFT JOIN authors a ON b.author_id = a.id; NULL if no match
| id | name | |----|---------| | 1 | Alice | | 2 | Bob | | 3 | Charlie |