SQL Fundamentals Quiz — Quiz

Question 1 of 10

Which clause filters groups after aggregation?

aHAVINGbWHEREcGROUP BYdORDER BY

Question 2 of 10

Which JOIN returns all rows from the left table?

aINNER JOINbCROSS JOINcLEFT JOINdRIGHT JOIN

Question 3 of 10

What does `COUNT(*)` count?

aNon-null valuesbDistinct valuescColumnsdAll rows

Question 4 of 10

Which keyword removes duplicate rows?

aUNIQUEbDISTINCTcDIFFERENTdSINGLE

Question 5 of 10

Which is best for storing money in MySQL?

aDECIMALbFLOATcDOUBLEdVARCHAR

Question 6 of 10

What does this return?

SELECT COALESCE(NULL, 0, 5);

aNULLb5cErrord0

Question 7 of 10

What does this query return?

SELECT name FROM staff
ORDER BY salary DESC
LIMIT 3 OFFSET 2;

aThe three lowest salariesbThe 3rd, 4th and 5th highest-paid staffcThe three highest-paid staffdThree rows in random order

Question 8 of 10

In MySQL 8, how does `TRUNCATE TABLE orders` differ from `DELETE FROM orders`?

aTRUNCATE only removes rows added todaybTRUNCATE keeps the rows but hides themcTRUNCATE drops and recreates the table, resets AUTO_INCREMENT and cannot be rolled backdThere is no difference

Question 9 of 10

Why does this query fail?

SELECT name FROM staff
WHERE salary > AVG(salary);

aAggregate functions are not allowed in WHERE; compare against a subquery such as (SELECT AVG(salary) FROM staff)bAVG only works on integer columnscThe query needs GROUP BY name firstdAVG must be spelled AVERAGE in MySQL

Question 10 of 10

Why do customers with no orders disappear from this result?

SELECT c.name, o.id
FROM customers c
LEFT JOIN orders o ON o.customer_id = c.id
WHERE o.status = 'PAID';

aLEFT JOIN cannot be combined with WHEREbThe WHERE test on the right table drops the NULL rows, so the join behaves like an INNER JOIN; move the condition into ONcThe query is missing GROUP BYdBecause `o.id` is selected