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.

Practice

1. Which Redis data structure is best for quickly checking if an item exists without caring about order?

easy

A. Set

B. Sorted Set

C. List

D. Hash

Solution

1. Step 1: Understand the purpose of Sets

   Sets store unique items and allow fast membership checks without any order.

2. Step 2: Compare with Sorted Sets

   Sorted Sets also store unique items but maintain order using scores, which is unnecessary if order doesn't matter.

3. Final Answer:

   Set -> Option A

4. Quick Check:

   Membership without order = Set [OK]

Hint: Use Set for membership checks without order [OK]

Common Mistakes:

• Confusing Sorted Set with Set for membership
• Thinking List is best for membership
• Assuming Hash stores unique items like Set

2. Which Redis command correctly adds a member to a Set named myset?

easy

A. SADD myset member1

B. ZADD myset 1 member1

C. LPUSH myset member1

D. HSET myset member1 1

Solution

1. Step 1: Identify the command for adding to a Set

   The command SADD adds members to a Set in Redis.

2. Step 2: Check other commands

   ZADD is for Sorted Sets, LPUSH is for Lists, and HSET is for Hashes.

3. Final Answer:

   SADD myset member1 -> Option A

4. Quick Check:

   SADD adds to Set [OK]

Hint: Use SADD to add members to a Set [OK]

Common Mistakes:

• Using ZADD for Sets
• Using LPUSH which is for Lists
• Using HSET which is for Hashes

3. Given the commands:
SADD fruits apple banana orange
ZADD scores 10 alice 20 bob 15 carol
Which command checks if 'banana' is a member of the Set fruits?

medium

A. SISMEMBER scores banana

B. ZSCORE fruits banana

C. ZRANK fruits banana

D. SISMEMBER fruits banana

Solution

1. Step 1: Identify the correct membership check for Sets

   SISMEMBER checks if an item is in a Set.

2. Step 2: Match the key and member

   The Set is fruits, and we check for 'banana'. So SISMEMBER fruits banana is correct.

3. Final Answer:

   SISMEMBER fruits banana -> Option D

4. Quick Check:

   Check membership in Set = SISMEMBER [OK]

Hint: Use SISMEMBER for Set membership checks [OK]

Common Mistakes:

• Using ZSCORE or ZRANK on Sets
• Checking membership on wrong key
• Confusing Sorted Set commands with Set commands

4. You want to check if 'alice' is in a Sorted Set scores. Which command is incorrect and will cause an error?

medium

A. ZRANK scores alice

B. ZSCORE scores alice

C. SISMEMBER scores alice

D. ZREM scores alice

Solution

1. Step 1: Understand commands for Sorted Sets

   ZSCORE, ZRANK, and ZREM are valid Sorted Set commands.

2. Step 2: Identify the invalid command

   SISMEMBER is only for Sets, not Sorted Sets, so it will cause an error.

3. Final Answer:

   SISMEMBER scores alice -> Option C

4. Quick Check:

   SISMEMBER invalid for Sorted Set [OK]

Hint: SISMEMBER works only on Sets, not Sorted Sets [OK]

Common Mistakes:

• Using SISMEMBER on Sorted Sets
• Confusing ZREM with SREM
• Assuming SISMEMBER works on all collections

5. You have a leaderboard stored as a Sorted Set leaderboard with player scores. You want to check if a player john exists in the leaderboard and get his rank. Which sequence of commands is correct?

hard

A. ZSCORE leaderboard johnSISMEMBER leaderboard john

B. ZSCORE leaderboard johnZRANK leaderboard john

C. ZRANK leaderboard johnSISMEMBER leaderboard john

D. SISMEMBER leaderboard johnZRANK leaderboard john

Solution

1. Step 1: Check existence and score in Sorted Set

   ZSCORE returns the score if the member exists, otherwise null.

2. Step 2: Get the rank if member exists

   ZRANK returns the rank of the member in the Sorted Set.

3. Step 3: Validate command order

   First check score with ZSCORE, then get rank with ZRANK. Using SISMEMBER is invalid for Sorted Sets.

4. Final Answer:

   ZSCORE leaderboard john
   ZRANK leaderboard john -> Option B

5. Quick Check:

   Use ZSCORE then ZRANK for Sorted Set membership and rank [OK]

Hint: Use ZSCORE then ZRANK for Sorted Set membership and rank [OK]

Common Mistakes:

• Using SISMEMBER on Sorted Sets
• Checking rank before confirming existence
• Mixing Set commands with Sorted Set commands