ZRANGE and ZREVRANGE for reading in Redis

Introduction

ZRANGE and ZREVRANGE help you get items from a sorted list in order or reverse order. This is useful when you want to see top scores or recent items.

You want to see the top 5 players in a game leaderboard.

You want to get the latest 10 messages sorted by time.

You want to list products sorted by price from low to high or high to low.

You want to read a range of dates from a timeline in order.

You want to fetch items ranked by popularity in reverse order.

Syntax

Redis

ZRANGE key start stop [WITHSCORES]
ZREVRANGE key start stop [WITHSCORES]

start and stop are zero-based indexes; 0 is the first item.

WITHSCORES option returns the scores along with the items.

Examples

Get the first 5 items from the sorted set 'leaderboard' in ascending order.

Redis

ZRANGE leaderboard 0 4

Get the top 3 items from 'leaderboard' in descending order with their scores.

Redis

ZREVRANGE leaderboard 0 2 WITHSCORES

Get items ranked 2nd to 4th from 'products' sorted set.

Redis

ZRANGE products 1 3

Sample Program

First, we add 5 players with their scores to 'leaderboard'. Then we get the lowest 3 scores with ZRANGE and the highest 3 scores with ZREVRANGE, both showing scores.

Redis

ZADD leaderboard 100 "Alice" 200 "Bob" 150 "Charlie" 120 "Diana" 180 "Eve"
ZRANGE leaderboard 0 2 WITHSCORES
ZREVRANGE leaderboard 0 2 WITHSCORES

Important Notes

Indexes are inclusive, so stop index is included in the result.

If stop is -1, it means the last element in the sorted set.

Without WITHSCORES, only the items are returned, not their scores.

Summary

ZRANGE gets items from lowest to highest score by index range.

ZREVRANGE gets items from highest to lowest score by index range.

Use WITHSCORES to see the scores along with the items.

Practice

1. What does the Redis command ZRANGE myset 0 2 do?

easy

A. Deletes the first three elements from the sorted set 'myset'.

B. Returns the first three elements with the lowest scores from the sorted set 'myset'.

C. Returns the last three elements with the highest scores from the sorted set 'myset'.

D. Returns all elements from the sorted set 'myset' without any order.

Solution

1. Step 1: Understand ZRANGE command behavior

   ZRANGE returns elements in ascending order by score, from the start index to the end index.

2. Step 2: Apply the index range 0 to 2

   Indexes 0 to 2 mean the first three elements with the lowest scores are returned.

3. Final Answer:

   Returns the first three elements with the lowest scores from the sorted set 'myset'. -> Option B

4. Quick Check:

   ZRANGE with 0 2 = first 3 lowest scored items [OK]

Hint: ZRANGE gets items low to high by index range [OK]

Common Mistakes:

• Confusing ZRANGE with ZREVRANGE order
• Thinking it deletes elements
• Assuming it returns all elements unordered

2. Which of the following is the correct syntax to get the top 5 highest scored elements from a sorted set named 'scores'?

easy

A. ZREVRANGE scores 0 4

B. ZRANGE scores -5 -1

C. ZRANGE scores 0 4 WITHSCORES

D. ZREVRANGE scores 5 0

Solution

1. Step 1: Identify command for highest scores first

   ZREVRANGE returns elements from highest to lowest score.

2. Step 2: Use index range 0 to 4 for top 5 elements

   Indexes 0 to 4 select the first 5 elements in descending order.

3. Final Answer:

   ZREVRANGE scores 0 4 -> Option A

4. Quick Check:

   Top 5 highest = ZREVRANGE 0 4 [OK]

Hint: Use ZREVRANGE 0 N-1 for top N highest scores [OK]

Common Mistakes:

• Using negative indexes incorrectly with ZRANGE
• Confusing ZRANGE and ZREVRANGE
• Wrong index order like 5 0

3. Given a sorted set 'players' with elements and scores: {"Alice":10, "Bob":20, "Carol":15}, what is the output of ZRANGE players 0 1 WITHSCORES?

medium

A. ["Bob", "20", "Carol", "15"]

B. ["Alice", "10", "Bob", "20"]

C. ["Carol", "15", "Bob", "20"]

D. ["Alice", "10", "Carol", "15"]

Solution

1. Step 1: Sort elements by score ascending

   Sorted order by score: Alice(10), Carol(15), Bob(20).

2. Step 2: Select indexes 0 to 1 with scores

   Indexes 0 and 1 correspond to Alice and Carol with their scores.

3. Final Answer:

   ["Alice", "10", "Carol", "15"] -> Option D

4. Quick Check:

   ZRANGE 0 1 WITHSCORES = Alice and Carol with scores [OK]

Hint: ZRANGE returns low to high; WITHSCORES adds scores after each element [OK]

Common Mistakes:

• Mixing order of elements
• Forgetting WITHSCORES doubles output
• Assuming ZREVRANGE order

4. You run ZREVRANGE leaderboard 0 2 but get unexpected results. Which of these is a likely cause?

medium

A. ZREVRANGE does not support index ranges.

B. You used ZRANGE instead of ZREVRANGE.

C. The sorted set 'leaderboard' is empty or does not exist.

D. The index range 0 2 is invalid for ZREVRANGE.

Solution

1. Step 1: Check if sorted set exists and has elements

   If 'leaderboard' is empty or missing, ZREVRANGE returns empty result.

2. Step 2: Validate command and index usage

   ZREVRANGE supports index ranges; 0 2 is valid; using ZRANGE instead would change order but not cause empty result.

3. Final Answer:

   The sorted set 'leaderboard' is empty or does not exist. -> Option C

4. Quick Check:

   Empty set causes empty ZREVRANGE output [OK]

Hint: Empty or missing set returns empty list [OK]

Common Mistakes:

• Thinking index range is invalid
• Confusing ZRANGE and ZREVRANGE effects
• Believing ZREVRANGE lacks index support

5. You want to get the middle 3 elements by score from a sorted set 'scores' with 10 elements, ordered from lowest to highest. Which command correctly retrieves elements ranked 4th to 6th (0-based index) with their scores?

hard

A. ZRANGE scores 3 5 WITHSCORES

B. ZREVRANGE scores 3 5 WITHSCORES

C. ZRANGE scores 4 6 WITHSCORES

D. ZREVRANGE scores 4 6 WITHSCORES

Solution

1. Step 1: Identify index range for middle 3 elements

   Elements ranked 4th to 6th correspond to indexes 3, 4, 5 (0-based).

2. Step 2: Use ZRANGE for ascending order and WITHSCORES to get scores

   ZRANGE scores 3 5 WITHSCORES returns elements at indexes 3 to 5 with their scores.

3. Final Answer:

   ZRANGE scores 3 5 WITHSCORES -> Option A

4. Quick Check:

   Middle 3 elements = ZRANGE 3 5 WITHSCORES [OK]

Hint: ZRANGE with correct indexes gets middle elements low to high [OK]

Common Mistakes:

• Using ZREVRANGE which reverses order
• Off-by-one errors in index range
• Confusing rank with index