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SQL for Beginners: Ask Your Data Straight Questions
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SQL for Beginners: Ask Your Data Straight Questions

Final Check: SQL for Beginners

Fifteen scenario questions across the whole course. If you get twelve or more right, you can read a small database, change it safely and combine its tables without silent mistakes.

15 min SQLDatabases Beginner
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1. A table needs a primary key for people, and you can choose an email address or a number the database creates. Which is better, and why?
The generated number, because it never changes when a person updates their email. The email, because it is already unique today. The email, because text keys are faster. Neither. Tables work without a primary key.
2. A colleague posts a query written for SQL Server that uses SELECT TOP 5. How do you limit rows to five in PostgreSQL?
Write HAVING 5. Write LIMIT 5 at the end, or FETCH FIRST 5 ROWS ONLY. Keep TOP 5. PostgreSQL reads it the same way. Write WHERE 5.
3. You page through products with LIMIT 3 OFFSET 3 and no ORDER BY, and page 2 repeats a row from page 1. What is the fix?
Use LIKE instead of LIMIT. Add an ORDER BY on a unique column, so the rows have a fixed order. Change LIMIT to a smaller number. Increase the OFFSET by one.
4. The product Gel Pen exists. SELECT name FROM products WHERE name LIKE 'g%' returns nothing in PostgreSQL. Why, and what would match it?
The percent sign must come first. LIKE is case sensitive, and the name starts with a capital G. ILIKE would match it. There is no product starting with a G. LIKE only works on numbers.
5. You write WHERE city NOT IN ('Cebu', NULL) and get zero rows, though many customers live elsewhere. What is the cause?
The list must be sorted. A NULL in a NOT IN list makes the result unknown for every row. NOT IN cannot be used with text. PostgreSQL ignores the second value.
6. A report must show every customer, with the text unknown where the city is missing, without changing the data. Which expression does it?
city = 'unknown' DELETE city COALESCE(city, 'unknown') UPDATE customers SET city = 'unknown'
7. A table has CHECK (percent_off BETWEEN 1 AND 100). What happens to INSERT ... VALUES ('BIGSALE', 150)?
The row is stored, but marked inactive. The row is refused with an error that names the check constraint. The row is stored with the value changed to 100. The row is stored and a warning is logged.
8. You store product prices in a double precision column, and a total is off by a fraction of a cent. What is the best fix?
Round every total with a larger number of decimals. Store the prices as text and add them in your program. Use integer and ignore the cents. Change the column to numeric(8,2), which stores exact decimals.
9. You are about to run UPDATE products SET price = price * 2 on a database you care about. What is the safest way to try it?
Run it and check afterwards. Type DELETE first to clear the old rows. Add LIMIT 1 so only one row is touched. Open a transaction with BEGIN, run it, check the result and ROLLBACK if it is wrong.
10. In a table of 20 rows, 5 have a NULL in the column score. What do count(*) and count(score) return?
20 and 15 15 and 15 20 and 20 15 and 20
11. You need customers who placed at least 3 orders that were not cancelled. Which query is correct?
GROUP BY customer_id with WHERE count(*) >= 3. HAVING status <> 'cancelled' with no GROUP BY. ORDER BY count(*) >= 3. WHERE status <> 'cancelled' to filter rows, GROUP BY customer_id, then HAVING count(*) >= 3.
12. SELECT city, count(*) FROM customers GROUP BY city returns four rows, one with an empty city. What is that row?
A customer whose city is an empty string. The customers whose city is NULL, grouped together. A row that PostgreSQL adds to show the total. A mistake in the data that the database kept.
13. You want every customer listed with the number of orders they placed, and customers with no orders must show 0. Which join do you start from?
customers LEFT JOIN orders, then count the order ids from the orders side. customers INNER JOIN orders. orders LEFT JOIN customers. customers CROSS JOIN orders.
14. You write FROM customers, orders and forget the matching condition. What do you get from 8 customers and 12 orders, and how would you have caught it?
96 rows, every customer paired with every order. Counting rows after each join shows the jump. 12 rows, because PostgreSQL matches the ids by itself. 8 rows, because the smaller table wins. An error, because every join needs an ON clause.
15. A query copied from an old MySQL answer selects customer_id, status and count(*) but groups only by customer_id. It ran there, and PostgreSQL stops it with an error. What does PostgreSQL want?
Nothing. The error is a bug in PostgreSQL. Replace GROUP BY with ORDER BY. Add status to GROUP BY, or remove it from the SELECT list, or wrap it in an aggregate. Turn the query into an UPDATE.

About

Fifteen scenario questions across the whole course. If you get twelve or more right, you can read a small database, change it safely and combine its tables without silent mistakes.

Duration
15 min
Last update
10/01/2026
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