Top-N queries in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Retrieve top 3 highest scores from a sorted set

Given a Redis sorted set game_scores with player scores, which command returns the top 3 players with the highest scores?

Redis

ZADD game_scores 50 player1 70 player2 60 player3 90 player4 80 player5

A ZREVRANGE game_scores 0 2 WITHSCORES

B ZRANGEBYSCORE game_scores 0 3 WITHSCORES

C ZRANGE game_scores 0 2 WITHSCORES

D ZREVRANGEBYSCORE game_scores 3 0 WITHSCORES

💡 Hint

Remember that sorted sets are ordered by score ascending by default. Use the command that retrieves elements in descending order.

ZRANGE returns elements with lowest scores first. ZREVRANGE returns elements with highest scores first. To get top 3 highest scores, use ZREVRANGE with indexes 0 to 2.

📝 Syntax

intermediate

Identify the correct syntax to get top 5 elements by score

Which of the following Redis commands correctly retrieves the top 5 elements with the highest scores from a sorted set named leaderboard?

A ZRANGE leaderboard 0 5

B ZRANGEBYSCORE leaderboard 5 -inf

C ZREVRANGE leaderboard 1 5

D ZREVRANGE leaderboard 0 4

💡 Hint

Indexes in Redis commands start at 0 and are inclusive. Use the command that returns elements in descending order.

ZREVRANGE leaderboard 0 4 returns elements ranked 1 to 5 by highest score. Other options either use wrong indexes or wrong commands.

❓ optimization

advanced

Efficiently retrieve top 10 elements with scores above 50

You want to get the top 10 elements from a sorted set scores where scores are greater than 50. Which command is the most efficient and correct?

A ZRANGEBYSCORE scores 50 +inf LIMIT 0 10

B ZREVRANGEBYSCORE scores +inf 50 LIMIT 0 10

C ZREVRANGEBYSCORE scores 50 +inf LIMIT 0 10

D ZRANGEBYSCORE scores +inf 50 LIMIT 0 10

💡 Hint

To get top elements with scores above 50, use reverse range by score from highest to lowest.

ZREVRANGEBYSCORE with max +inf and min 50 returns elements with scores >= 50 in descending order. LIMIT 0 10 restricts to top 10. Other options either use wrong order or wrong min/max.

🔧 Debug

advanced

Why does this top-N query return empty?

You run the command ZREVRANGEBYSCORE players 100 50 LIMIT 0 5 on a sorted set players but get an empty result. What is the reason?

A The sorted set <code>players</code> is empty.

B The LIMIT clause is invalid and causes no results.

C The min and max scores are reversed; max should be greater than min.

D ZREVRANGEBYSCORE does not support LIMIT.

💡 Hint

Check the order of min and max scores in the command.

ZREVRANGEBYSCORE expects max score first, then min score. Here 100 is max and 50 is min, which is correct. But if the scores are reversed, it returns empty. The command is correct but if the scores are reversed, no results appear.

🧠 Conceptual

expert

Understanding Top-N queries with Redis sorted sets and lexicographical order

You have a sorted set users where scores are all equal (e.g., 0), but you want to get the top 5 users sorted by their names alphabetically. Which command achieves this?

A ZRANGEBYLEX users - + LIMIT 0 5

B ZRANGEBYSCORE users 0 0 LIMIT 0 5

C ZREVRANGEBYLEX users + - LIMIT 0 5

D ZREVRANGEBYSCORE users 0 0 LIMIT 0 5

💡 Hint

Use lexicographical range commands when scores are equal.

ZRANGEBYLEX returns elements in lex order when scores are equal. '-' and '+' represent min and max lex range. LIMIT 0 5 returns first 5 elements alphabetically. Other options use score ranges or reverse order which do not sort lexicographically ascending.