Top-N queries in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to get the top 3 scores from the sorted set 'game_scores'.

Redis

ZREVRANGE game_scores 0 [1] WITHSCORES

Drag options to blanks, or click blank then click option'

A4

B3

C2

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 3 as stop index returns 4 elements, not 3.

Using -1 returns all elements, not top 3.

The command ZREVRANGE with start 0 and stop 2 returns the top 3 elements (0-based index).

2fill in blank

medium

Complete the code to get the top 5 scores in descending order from 'game_scores'.

Redis

ZREVRANGE game_scores 0 [1] WITHSCORES

Drag options to blanks, or click blank then click option'

A4

B5

C3

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 5 as stop index returns 6 elements.

Using ZRANGE instead of ZREVRANGE returns lowest scores first.

ZREVRANGE returns elements in descending order. To get top 5, use stop index 4.

3fill in blank

hard

Fix the error in the code to get the top 3 scores with their members from 'game_scores'.

Redis

ZREVRANGE game_scores 0 [1] WITHSCORES

Drag options to blanks, or click blank then click option'

A-1

B2

C-3

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Using 3 returns 4 elements.

Using negative indexes may cause unexpected results.

The stop index should be 2 to get exactly 3 elements (0,1,2).

4fill in blank

hard

Fill both blanks to get the top 4 scores with their members in descending order from 'game_scores'.

Redis

ZREVRANGE game_scores [1] [2] WITHSCORES

Drag options to blanks, or click blank then click option'

A0

B3

C4

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 4 as stop index returns 5 elements.

Using -1 as stop returns all elements.

Start at 0 and stop at 3 to get top 4 elements (0,1,2,3) in descending order.

5fill in blank

hard

Fill all three blanks to get the top 3 scores with their members from 'game_scores' using ZREVRANGEBYSCORE for scores between 100 and 200.

Redis

ZREVRANGEBYSCORE game_scores [1] [2] LIMIT 0 [3] WITHSCORES

Drag options to blanks, or click blank then click option'

A100

B200

C3

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping min and max values.

Using wrong limit count.

The command gets members with scores between 100 and 200, limiting to top 3 results.

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