SQL Filtering and Sorting Practice

Practise DISTINCT, ORDER BY, LIMIT, AND/OR/NOT, NULL checks and column aliases on a product catalogue.

Beginner8 problemsUpdated October 23, 2026

Review first: DISTINCT, ORDER BY, LIMIT, AND / OR / NOT, NULL Values, Aliases

All problems use this products table:

product_id	name	category	price	stock	supplier
1	Laptop	Electronics	55000	12	TechWorld
2	Headphones	Electronics	2500	40	SoundCo
3	Desk Chair	Furniture	7000	8	NULL
4	Notebook	Stationery	60	500	PaperPlus
5	Monitor	Electronics	12000	0	TechWorld
6	Bookshelf	Furniture	4500	5	NULL
7	Pen Set	Stationery	250	120	PaperPlus
8	Keyboard	Electronics	1800	25	TechWorld

Try your answers on these tables in the SQL Playground

1.List the categories
Beginner
Return each product category once, in alphabetical order.
Show hint
DISTINCT removes duplicate rows; ORDER BY sorts them.
Show solution
SQLRun
SELECT DISTINCT category FROM products ORDER BY category;
Output
category ----------- Electronics Furniture Stationery
2.Three most expensive products
Beginner
Return the name and price of the three most expensive products, most expensive first.
Show hint
Sort by price from high to low, then keep only the first three rows.
Show solution
SQLRun
SELECT name, price FROM products ORDER BY price DESC LIMIT 3;
Output
name | price -----------+------ Laptop | 55000 Monitor | 12000 Desk Chair | 7000
LIMIT works in PostgreSQL, MySQL and SQLite. SQL Server writes SELECT TOP 3 name, price ..., and Oracle (and standard SQL) adds FETCH FIRST 3 ROWS ONLY after ORDER BY. Without ORDER BY, "the first three rows" could be any three rows.
3.The second page of results
Beginner
A shop shows 3 products per page, sorted by price from high to low. Return the name and price of the products on page 2.
Show hint
Page 2 skips the first 3 rows. Combine LIMIT with OFFSET.
Show solution
SQLRun
SELECT name, price FROM products ORDER BY price DESC LIMIT 3 OFFSET 3;
Output
name | price -----------+------ Bookshelf | 4500 Headphones | 2500 Keyboard | 1800
For page n with k rows per page, use LIMIT k OFFSET (n - 1) * k. The ORDER BY is essential: without a fixed order, pages can overlap or skip rows.
4.Affordable electronics in stock
Beginner
Return the name, price and stock of Electronics products that are in stock and cost less than 20000. Sort by price.
Show hint
All three conditions must be true, so join them with AND.
Show solution
SQLRun
SELECT name, price, stock FROM products WHERE category = 'Electronics' AND stock > 0 AND price < 20000 ORDER BY price;
Output
name | price | stock -----------+-------+------ Keyboard | 1800 | 25 Headphones | 2500 | 40
The Monitor costs less than 20000 but has a stock of 0, so the stock > 0 condition removes it.
5.AND, OR and parentheses
Beginner
Return the name, category and price of products that are Electronics or Furniture and cost under 5000. Sort by price.
Show hint
AND is evaluated before OR. Use parentheses around the two category checks.
Show solution
SQLRun
SELECT name, category, price FROM products WHERE (category = 'Electronics' OR category = 'Furniture') AND price < 5000 ORDER BY price;
Output
name | category | price -----------+-------------+------ Keyboard | Electronics | 1800 Headphones | Electronics | 2500 Bookshelf | Furniture | 4500
Without the parentheses, the condition is read as category = 'Electronics' OR (category = 'Furniture' AND price < 5000), which wrongly returns every Electronics product, including the Laptop and the Monitor. category IN ('Electronics', 'Furniture') is a shorter way to write the same check.
6.Products with no supplier
Beginner
Return the name of every product whose supplier is unknown (NULL).
Show hint
= NULL is never true. There is a special operator for NULL checks.
Show solution
SQLRun
SELECT name FROM products WHERE supplier IS NULL ORDER BY name;
Output
name ---------- Bookshelf Desk Chair
Comparing anything with NULL using = gives unknown, not true, so WHERE supplier = NULL returns no rows at all. Always use IS NULL and IS NOT NULL.

7.Replace NULL with a label

Beginner

Return every product's name and its supplier as a column called supplier_name, showing Unknown where the supplier is NULL. Sort by product name.

Show hint

COALESCE returns its first argument that is not NULL.

Show solution

SQLRun

SELECT name, COALESCE(supplier, 'Unknown') AS supplier_name
FROM products
ORDER BY name;

Output

name       | supplier_name
-----------+--------------
Bookshelf  | Unknown
Desk Chair | Unknown
Headphones | SoundCo
Keyboard   | TechWorld
Laptop     | TechWorld
Monitor    | TechWorld
Notebook   | PaperPlus
Pen Set    | PaperPlus

8.Most valuable stock
Beginner
The value of a product's stock is price * stock. Return the product name as product and that value as stock_value for the three products with the highest stock value.
Show hint
Give the calculation an alias with AS, then sort by the alias.
Show solution
SQLRun
SELECT name AS product, price * stock AS stock_value FROM products ORDER BY stock_value DESC LIMIT 3;
Output
product | stock_value -----------+------------ Laptop | 660000 Headphones | 100000 Desk Chair | 56000
A column alias can be used in ORDER BY because ORDER BY runs after SELECT. It cannot be used in WHERE, which runs before SELECT.

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