Set vs sorted set for membership in Redis

Introduction

Sets and sorted sets help you keep track of groups of items. You use them to check if something is part of a group quickly.

You want to store a list of unique items and check if an item is in that list.

You need to keep items in order by a score and check if an item is in the list.

You want to count how many items are in a group.

You want to get items sorted by a number like a score or time.

You want to add or remove items from a group easily.

Syntax

Redis

SADD key member [member ...]
SISMEMBER key member
ZADD key [NX|XX] [CH] [INCR] score member [score member ...]
ZRANK key member

Sets store unique members without order.

Sorted sets store unique members with a score to keep them ordered.

Examples

Add fruits to a set and check if 'banana' is in the set.

Redis

SADD myset apple banana cherry
SISMEMBER myset banana

Add people with scores to a sorted set and find the rank of 'bob'.

Redis

ZADD myzset 10 alice 20 bob 15 carol
ZRANK myzset bob

Sample Program

This example adds fruits to a set and checks membership. It also adds players with scores to a sorted set and gets the rank of 'bob'.

Redis

SADD fruits apple banana cherry
SISMEMBER fruits banana
ZADD leaderboard 100 alice 200 bob 150 carol
ZRANK leaderboard bob

Important Notes

SISMEMBER returns 1 if the member exists, 0 if not.

ZRANK returns the zero-based rank of the member or nil if not found.

Sets are faster for simple membership checks without order.

Summary

Sets store unique items without order and are good for quick membership checks.

Sorted sets store unique items with scores to keep them ordered.

Use sets when order does not matter, and sorted sets when you need order or ranking.

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