How do I remove duplicate rows?

Rank the rows inside each duplicate group with ROW_NUMBER and keep number 1. PARTITION BY the columns that define a duplicate, such as email. ORDER BY decides which row survives, for example the newest updated_at. SELECT DISTINCT only removes rows that are identical in every column.

How do I fix inconsistent text values?

Normalise text before you group, join or compare it. TRIM removes stray spaces; LOWER or UPPER fixes case. REPLACE swaps fixed characters; CAST fixes types. Group by the cleaned expression, or "Pune", "pune " and "PUNE" count as three cities.

SQL Practice Hub

SQL Data Cleaning Practice Questions

Leaderboard

Fix missing values, duplicates and messy formatting before you analyse.

6 questionsFreeRuns in your browser

NULL

6 questions

COALESCE: Handle Missing Composers
Many tracks have NULL composers. Use COALESCE to provide a default value when the Composer is missing. Write a query...
COALESCENULL Handling
Beginner
4m15 pts
16100%
Handle NULL Company Names
Some customers don't have a company. Use COALESCE to show 'Individual' for customers without a company. Write a query...
COALESCENULL Handling
Beginner
4m15 pts
16100%
Self-Join: Customers in Same City
Find pairs of customers who live in the same city. This can help with organizing local meetups or referral programs....
Self-JoinJOINDeduplication
Intermediate
9m25 pts
1594%
LEFT JOIN: All Artists Including Those Without Albums
An INNER JOIN only returns matching rows. Use LEFT JOIN to include ALL artists, even those without any albums. Write a...
LEFT JOINAggregationNULL Handling
Intermediate
6m25 pts
1493%
North America Customer Profile
The customer success team wants a quick contact list of customers from North America, showing each as a single readable...
WHEREINConcatenationCOALESCE+3
Intermediate
8m20 pts
267%
Customer Email Domain Report
The analytics team wants a quick view of customer email providers, alongside each customer's full name and an...
String FunctionsSUBSTRSTRPOSConcatenation+5
Intermediate
10m25 pts
267%

FAQ

SQL Data Cleaning practice: common questions

Short answers to what people get stuck on most. Tap a question to expand it.

Why does = NULL not work?

NULL means unknown, so NULL = NULL is not true. Test with IS NULL instead.

COALESCE(col, default) replaces NULL with a fallback value.
NULLIF(col, '') turns an empty string into NULL.
Most aggregates ignore NULL; COUNT(*) does not.

Track your streak & earn XP

Full-Length Interview Practice Tests100% Free

10 role-based tests for Data Analysts, SQL Developers & Data Engineers with instant scorecards.

Explore 10 Mock Tests

SQL Data Cleaning Practice Questions

COALESCE: Handle Missing Composers

Handle NULL Company Names

Self-Join: Customers in Same City

LEFT JOIN: All Artists Including Those Without Albums

North America Customer Profile

Customer Email Domain Report

SQL Data Cleaning practice: common questions

Full-Length Interview Practice Tests100% Free

SQL Data Cleaning Practice Questions

COALESCE: Handle Missing Composers

Handle NULL Company Names

Self-Join: Customers in Same City

LEFT JOIN: All Artists Including Those Without Albums

North America Customer Profile

Customer Email Domain Report

SQL Data Cleaning practice: common questions

Full-Length Interview Practice Tests100% Free

What you will practice

Related practice topics