Question bank

Pick an answer to see if you're right — with an explanation.

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SQL / MySQLEasy

Which clause filters groups after aggregation?

aHAVING

bWHERE

cGROUP BY

dORDER BY

SQL / MySQLEasy

Which keyword starts a Common Table Expression?

aWITH

bUSING

cDEFINE

dAS

SQL / MySQLEasy

Which JOIN returns all rows from the left table?

aINNER JOIN

bCROSS JOIN

cLEFT JOIN

dRIGHT JOIN

SQL / MySQLEasy

Which statement permanently saves the changes made in a transaction?

aSAVE

bROLLBACK

cCOMMIT

dEND

SQL / MySQLEasy

What does `COUNT(*)` count?

aNon-null values

bDistinct values

cColumns

dAll rows

SQL / MySQLEasy

Which window function gives the value from the previous row?

aLEAD

bFIRST_VALUE

cPREV

dLAG

SQL / MySQLEasy

Which keyword removes duplicate rows?

aUNIQUE

bDISTINCT

cDIFFERENT

dSINGLE

SQL / MySQLEasy

In `EXPLAIN` output, what does `type = ALL` indicate?

aAll indexes were used

bA full table scan

cThe query returns all columns

dThe result is cached

SQL / MySQLMedium

Which is best for storing money in MySQL?

aDECIMAL

bFLOAT

cDOUBLE

dVARCHAR

SQL / MySQLMedium

Scores are 90, 80, 80, 70. What `DENSE_RANK()` (ORDER BY score DESC) does 70 get?

SQL / MySQLMedium

What does this return?

SELECT COALESCE(NULL, 0, 5);

aNULL

cError

SQL / MySQLMedium

Why can this query return zero rows even though some customers never ordered?

SELECT name FROM customers
WHERE id NOT IN (SELECT customer_id FROM orders);

aNOT IN only works with numbers

bThe subquery must use DISTINCT

cIf `orders.customer_id` contains a NULL, NOT IN evaluates to unknown for every row

dSubqueries cannot be used in WHERE

SQL / MySQLMedium

What does this query return?

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

aThe three lowest salaries

bThe 3rd, 4th and 5th highest-paid staff

cThe three highest-paid staff

dThree rows in random order

SQL / MySQLMedium

Which index can MySQL use for `WHERE status = 'PAID'` alone?

a(customer_id, status)

b(created_at, status)

c(customer_id, created_at, status)

d(status, created_at)

SQL / MySQLMedium

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

aTRUNCATE only removes rows added today

bTRUNCATE keeps the rows but hides them

cTRUNCATE drops and recreates the table, resets AUTO_INCREMENT and cannot be rolled back

dThere is no difference