SQL Database Relational Join Logic Quiz

A company wants to list only employees who belong to a department. Which join returns only rows with matching department records?

A retailer wants to show all products, even those without sales yet. Which join ensures products appear regardless of sales?

A user checks which orders lack assigned customers because of missing customer IDs. Which join helps reveal orders without a match?

A student portal wants to show all students and all courses, even if some combinations have no enrollment. Which join captures all rows from both sides?

A system lists employees and their managers from within the same table. What join technique compares rows of a table with itself?

A product list is paired with every available color option to form combinations. Which join intentionally forms all possible row pairs?

A report shows customers even if they have no orders, but only shows orders when they match a customer. Which join is at work?

A query compares student IDs from one table to matching records in another. Why is an ON condition required?

A developer notices NULL on the right side of a left join. What does this usually indicate?

In a comparison of right join and left join, which behavior uniquely belongs to right joins?

A dataset uses INNER JOIN when retrieving invoices and their customers. Why do unmatched invoices disappear from the result?

A store wants to show both customers with orders and customers without orders, while also showing orders without customers. Which join works?

A manufacturer uses a CROSS JOIN to list all material and size combinations. Why might the resulting table be very large?

Why must columns used in join comparisons generally share compatible data types?

A self join compares parent IDs to child IDs within one table. Why is table aliasing necessary?

Why might an inner join shrink the number of returned rows compared to the original tables?

A left join lists all customers and shows NULL for order details if none exist. Why is this helpful in reporting?

A cross join produces a pattern where every supplier is paired with every region. What is this pattern called?

A report shows suppliers with no products after a right join. Why?

A chain of joins connects employees → departments → locations. Why is join order important?

A query uses FULL JOIN to list all rows from both tables. Why might some fields show NULL?

A developer uses JOIN without specifying an ON condition. Why might the query produce unexpected results?

A query extracting customer orders uses a join condition on customer_id. Why must the values in customer_id be consistent?

A financial system uses INNER JOIN to combine transactions with accounts. Why might unmatched transactions be purposely excluded?

Why is a CROSS JOIN useful when generating all possible pairings between two sets, such as time slots and rooms?

What does this query show about orders and customers?

SELECT o.id, c.name FROM orders o INNER JOIN customers c ON o.customer_id = c.id;

This query keeps all customers even when order info is missing. What result appears?

SELECT c.id, o.id FROM customers c LEFT JOIN orders o ON c.id = o.customer_id;

What does this right join preserve?

SELECT p.name, r.region FROM products p RIGHT JOIN regions r ON p.region_id = r.id;

This full join mixes employees and departments. What rows appear?

SELECT e.name, d.name FROM employees e FULL JOIN departments d ON e.dept_id = d.id;

A cross join creates pairings between products and categories. What does the output look like?

SELECT p.name, c.type FROM products p CROSS JOIN categories c;

What does this join reveal about book authors?

SELECT b.title, a.name FROM books b INNER JOIN authors a ON b.author_id = a.id;

Which rows survive this filtering left join?

SELECT u.username, p.post FROM users u LEFT JOIN posts p ON u.id = p.user_id WHERE p.user_id IS NOT NULL;

This join links orders and shipments. What happens when a shipment is missing?

SELECT o.id, s.date FROM orders o LEFT JOIN shipments s ON o.id = s.order_id;

What output shape results from joining students to enrollments on student IDs?

SELECT s.name, e.course_id FROM students s INNER JOIN enrollments e ON s.id = e.student_id;

A query finds missing matches with this structure. What rows appear?

SELECT a.id, b.value FROM a LEFT JOIN b ON a.id = b.a_id WHERE b.a_id IS NULL;

What does this multi-join accomplish?

SELECT e.name, d.name, l.city FROM employees e INNER JOIN departments d ON e.dept_id = d.id INNER JOIN locations l ON d.loc_id = l.id;

This join lists suppliers and their products. How are unmatched suppliers represented?

SELECT s.name, p.name FROM suppliers s LEFT JOIN products p ON s.id = p.supplier_id;

A cross join mixes all days with all shifts. What is the result size?

SELECT d.day, s.shift FROM days d CROSS JOIN shifts s;

This join finds employees whose departments lack location records. What pattern is used?

SELECT e.id FROM employees e LEFT JOIN departments d ON e.dept_id = d.id WHERE d.id IS NULL;

What does this grouped join output represent?

SELECT c.country, COUNT(*) FROM customers c INNER JOIN orders o ON c.id = o.customer_id GROUP BY c.country;

Which rows appear after this join filters NULL matches?

SELECT c.name FROM categories c LEFT JOIN items i ON c.id = i.cat_id WHERE i.cat_id IS NOT NULL;

What is the effect of this join condition?

SELECT p.name, b.title FROM publishers p INNER JOIN books b ON p.id = b.publisher_id;

This full join connects customers and feedback. What rows are included?

SELECT c.id, f.comment FROM customers c FULL JOIN feedback f ON c.id = f.customer_id;

This self join compares employees by manager. What does it show?

SELECT e.name, m.name FROM employees e JOIN employees m ON e.manager_id = m.id;

This join helps detect orphan rows. What does it return?

SELECT o.id FROM orders o LEFT JOIN customers c ON o.customer_id = c.id WHERE c.id IS NULL;

A join links countries and regions. How are unmatched countries represented?

SELECT c.name, r.name FROM countries c LEFT JOIN regions r ON c.region_id = r.id;

This join enriches products with supplier data. What does the ON condition enforce?

SELECT p.name, s.contact FROM products p INNER JOIN suppliers s ON p.supplier_id = s.id;

This join simulates a calendar expansion. What does it produce?

SELECT d.date, s.slot FROM dates d CROSS JOIN slots s;

This join keeps all products and adds brand info when available. What does it show?

SELECT p.name, b.brand FROM products p LEFT JOIN brands b ON p.brand_id = b.id;

A company links events to locations. What does this inner join produce?

SELECT e.title, l.place FROM events e INNER JOIN locations l ON e.location_id = l.id;