SQL foundations: check what you know

Already know some SQL? Answer these 15 questions, about 8 minutes. Each topic you get fully right is marked Known on the SQL roadmap, so you can skip it. Each one you miss is listed for you to study.

Don’t guess. Pick “I don’t know yet” when you’re not sure: a lucky guess would mark a topic Known that you still need to learn.

The 10 topics it checks

1. Tables, rows and keys
2. SELECT, WHERE, ORDER BY and LIMIT
3. DISTINCT
4. Aggregates
5. GROUP BY and HAVING
6. NULL and three-valued logic
7. Joins: inner, left, right, full, self and cross
8. Simple subqueries
9. INSERT, UPDATE and DELETE
10. Data types and constraints

1. Question 1 of 15 · Predict the outputorders.customer_id has REFERENCES customers. There is no customer 99. What happens?

   INSERT INTO orders (order_id, customer_id, order_date, status, total)
   VALUES (115, 99, '2026-10-21', 'placed', 10.00);
   

   The order goes in, with customer_id 99.The order goes in, with customer_id NULL.A customer 99 is made for it.It stops with an error.I don’t know yet
2. Question 2 of 15 · Predict the outputThe store has 3 electronics products and 2 home ones. Of these 5, only the headphones (59.00) and the desk lamp (24.99) cost more than 20. How many rows?

   SELECT name
   FROM products
   WHERE category = 'electronics' OR category = 'home' AND price > 20;
   

   2451I don’t know yet
3. Question 3 of 15 · Predict the outputorders has 14 rows, from 7 customers, with 4 different statuses. How many rows?

   SELECT DISTINCT customer_id, status
   FROM orders;
   

   741214I don’t know yet
4. Question 4 of 15 · Predict the outputBen (customer 2) has 4 orders, two of them on 14 October. No one else has more than 3 orders. Which customers does this list?

   SELECT customer_id
   FROM orders
   GROUP BY customer_id
   HAVING count(DISTINCT order_date) >= 4;
   

   BenBen and AshaIt stops with an errorNo oneI don’t know yet
5. Question 5 of 15 · Spot the bugThis should list the customers with 2 or more orders. What happens?

   SELECT customer_id, count(*)
   FROM orders
   WHERE count(*) >= 2
   GROUP BY customer_id;
   

   It stops with an error.It works, and lists 4 customers.It counts every order as one group.It gives no rows.I don’t know yet
6. Question 6 of 15 · Predict the outputOf the 10 employees, Jon and Zoe have manager 7, and Meera has no manager (NULL). How many rows?

   SELECT name
   FROM employees
   WHERE manager_id <> 7;
   

   81079I don’t know yet
7. Question 7 of 15 · Predict the outputevents has 20 rows: 19 from the store’s customers, and a page view from a visitor who wasn’t signed in (customer_id is NULL). Of the 8 customers, only Chen has no events. How many rows?

   SELECT c.name, e.event_id
   FROM customers c
   FULL JOIN events e ON e.customer_id = c.customer_id;
   

   21201928I don’t know yet
8. Question 8 of 15 · Spot the bugThis should count each customer’s orders. What does it give Dana, who has none?

   SELECT c.name, count(*) AS orders
   FROM customers c
   LEFT JOIN orders o ON o.customer_id = c.customer_id
   GROUP BY c.customer_id, c.name;
   

   0NULLDana isn’t listed1I don’t know yet
9. Question 9 of 15 · Predict the outputOf the store’s 14 orders, only Ben’s 109 and 110 share a customer and a day. How many rows does this self join give?

   SELECT a.order_id, b.order_id
   FROM orders a
   JOIN orders b
     ON b.customer_id = a.customer_id
     AND b.order_date = a.order_date
     AND b.order_id <> a.order_id;
   

   121416I don’t know yet
10. Question 10 of 15 · Predict the outputBen (customer 2) has 4 orders. How many times is he listed?

    SELECT name
    FROM customers
    WHERE customer_id IN (SELECT customer_id FROM orders);
    

    Once4 timesNot at allIt stops with an errorI don’t know yet
11. Question 11 of 15 · Spot the bugThis should list the 6 employees who manage no one. Meera’s manager_id is NULL. How many rows does it give?

    SELECT name
    FROM employees
    WHERE employee_id NOT IN (SELECT manager_id FROM employees);
    

    61004I don’t know yet
12. Question 12 of 15 · Predict the outputpayments has 13 rows. What does the last line give?

    BEGIN;
    DELETE FROM payments WHERE payment_id = 13;
    ROLLBACK;
    SELECT count(*) FROM payments;
    

    12130It stops with an errorI don’t know yet
13. Question 13 of 15 · Predict the outputProduct 9, the phone stand, is already in products at 6.90. What is its price after this?

    INSERT INTO products (product_id, name, category, price)
    VALUES (9, 'Phone stand', 'electronics', 7.50)
    ON CONFLICT (product_id) DO UPDATE SET price = EXCLUDED.price;
    

    6.90It stops with an error7.50There are two rows: 6.90 and 7.50I don’t know yet
14. Question 14 of 15 · Multiple choiceWhich column type keeps a gift card balance exact to the cent?

    floatrealdouble precisionnumeric(10, 2)I don’t know yet
15. Question 15 of 15 · Multiple choiceA reviews table has stars integer NOT NULL CHECK (stars BETWEEN 1 AND 5). Which value of stars goes in?

    'five'30NULLI don’t know yet

See my results

Answered 0 of 15.