Set vs sorted set for membership in Redis - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

Difference in membership check performance

Which Redis data structure provides the fastest membership check for a single element?

A Both have the same performance since they store elements similarly.

B Sorted Set, because it maintains elements in order for quick binary search.

C Set, because it uses a hash table for O(1) membership checks.

D Neither supports membership checks efficiently; use a list instead.

💡 Hint

Think about how each data structure stores elements and how that affects lookup speed.

Redis Sets use hash tables, allowing O(1) time complexity for membership checks. Sorted Sets use skip lists and hashes but are optimized for range queries, not membership speed.

❓ query_result

intermediate

Check membership in a Redis Set

Given a Redis Set named fruits containing apple, banana, and cherry, what is the output of the command SISMEMBER fruits banana?

A nil

B 1

C 0

D Error

💡 Hint

Remember what SISMEMBER returns when the element exists in the set.

SISMEMBER returns 1 if the element is in the set, 0 if not.

📝 Syntax

advanced

Valid command to check membership in a Sorted Set

Which of the following Redis commands correctly checks if the member user42 exists in the Sorted Set online_users?

A ZMEMBER online_users user42

B SISMEMBER online_users user42

C ZRANK online_users user42

D ZSCORE online_users user42

💡 Hint

Think about which command returns a score or indication of membership for sorted sets.

ZSCORE returns the score of a member if it exists, otherwise nil. SISMEMBER is for sets, not sorted sets. ZRANK returns the rank or nil, which can also indicate membership but is less direct. ZMEMBER is not a valid command.

❓ optimization

advanced

Choosing data structure for frequent membership checks with ordering

You need to frequently check if elements exist and also retrieve them in a sorted order by score. Which Redis data structure is best suited?

A Sorted Set, because it supports fast membership checks and maintains order.

B Set, because it has faster membership checks and you can sort on retrieval.

C List, because it maintains order and supports membership checks efficiently.

D Hash, because it stores key-value pairs and supports membership checks.

💡 Hint

Consider both membership check speed and the need to keep elements sorted.

Sorted Sets provide ordered elements by score and allow membership checks via commands like ZSCORE. Sets have faster membership checks but no order. Lists and Hashes do not efficiently support both requirements.

🔧 Debug

expert

Why does this membership check fail on a Sorted Set?

You run the command SISMEMBER online_users user42 on a Sorted Set online_users and it returns an error even though user42 exists. Why?

A Because <code>SISMEMBER</code> only works on Sets, not Sorted Sets.

B Because the Sorted Set is empty.

C Because <code>user42</code> is not a member of the Sorted Set.

D Because <code>SISMEMBER</code> requires a score argument for Sorted Sets.

💡 Hint

Check command compatibility with data types.

SISMEMBER is designed for Sets only. For Sorted Sets, membership checks require commands like ZSCORE or ZRANK. Using SISMEMBER on a Sorted Set results in a WRONGTYPE error.