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.

Practice

1. What Redis command is used to get the top N highest scores from a sorted set?

easy

A. ZRANGE

B. ZREVRANGE

C. ZADD

D. ZREM

Solution

1. Step 1: Understand sorted set ordering

   Redis sorted sets store elements with scores. To get highest scores, we want descending order.

2. Step 2: Identify the correct command

   ZREVRANGE returns elements in descending order by score, perfect for top N queries.

3. Final Answer:

   ZREVRANGE -> Option B

4. Quick Check:

   Top N highest = ZREVRANGE [OK]

Hint: Use ZREVRANGE for top scores in descending order [OK]

Common Mistakes:

• Using ZRANGE which returns lowest scores first
• Confusing ZADD (adds elements) with query commands
• Using ZREM which removes elements

2. Which of the following is the correct syntax to get the top 3 highest scoring members from sorted set players?

easy

A. ZREVRANGE players 1 3

B. ZRANGE players 0 2

C. ZREVRANGE players 0 2

D. ZRANGE players -3 -1

Solution

1. Step 1: Understand index ranges in ZREVRANGE

   Indexes start at 0. To get top 3, use 0 to 2 inclusive.

2. Step 2: Choose correct command and range

   ZREVRANGE returns highest scores first. So ZREVRANGE players 0 2 gets top 3.

3. Final Answer:

   ZREVRANGE players 0 2 -> Option C

4. Quick Check:

   Top 3 highest = ZREVRANGE players 0 2 [OK]

Hint: Use 0 to N-1 indexes with ZREVRANGE for top N [OK]

Common Mistakes:

• Using ZRANGE which returns lowest scores
• Starting index at 1 instead of 0
• Using negative indexes incorrectly

3. Given a sorted set scores with members and scores: {"alice": 50, "bob": 70, "carol": 60}, what is the result of ZREVRANGE scores 0 1 WITHSCORES?

medium

A. ["carol", "60", "alice", "50"]

B. ["alice", "50", "carol", "60"]

C. ["bob", "70", "alice", "50"]

D. ["bob", "70", "carol", "60"]

Solution

1. Step 1: Order members by descending score

   Scores descending: bob(70), carol(60), alice(50).

2. Step 2: Select top 2 with scores

   ZREVRANGE 0 1 returns first two: bob and carol with their scores.

3. Final Answer:

   ["bob", "70", "carol", "60"] -> Option D

4. Quick Check:

   Top 2 with scores = bob(70), carol(60) [OK]

Hint: ZREVRANGE 0 N-1 gets top N in descending order [OK]

Common Mistakes:

• Mixing ascending and descending order
• Forgetting WITHSCORES returns pairs
• Selecting wrong index range

4. You run ZREVRANGE leaderboard 0 4 but get an empty list. What is the most likely cause?

medium

A. The sorted set leaderboard does not exist or is empty

B. The command syntax is incorrect

C. ZREVRANGE only works with hashes, not sorted sets

D. The indexes 0 to 4 are out of range for the sorted set

Solution

1. Step 1: Check command syntax validity

   ZREVRANGE syntax is correct for sorted sets, so syntax is not the issue.

2. Step 2: Understand empty result causes

   If the sorted set does not exist or is empty, ZREVRANGE returns an empty list.

3. Final Answer:

   The sorted set leaderboard does not exist or is empty -> Option A

4. Quick Check:

   Empty result means no data in sorted set [OK]

Hint: Empty list means sorted set missing or empty [OK]

Common Mistakes:

• Assuming syntax error without checking data
• Thinking ZREVRANGE works on hashes
• Believing index out of range causes error instead of empty list

5. You want to get the top 5 players with the lowest scores from a sorted set game_scores. Which command should you use?

hard

A. ZRANGE game_scores 0 4

B. ZRANGE game_scores -5 -1

C. ZREVRANGE game_scores -5 -1

D. ZREVRANGE game_scores 0 4

Solution

1. Step 1: Identify order for lowest scores

   ZRANGE returns elements in ascending order by score, so lowest scores first.

2. Step 2: Select correct range for top 5

   Indexes 0 to 4 select first 5 elements, which are lowest scores.

3. Final Answer:

   ZRANGE game_scores 0 4 -> Option A

4. Quick Check:

   Lowest 5 scores = ZRANGE 0 4 [OK]

Hint: Use ZRANGE 0 N-1 for lowest N scores [OK]

Common Mistakes:

• Using ZREVRANGE which returns highest scores
• Using negative indexes incorrectly
• Confusing ascending and descending order