How to Use SDIFF in Redis: Syntax and Examples

Use the SDIFF command in Redis to get members of the first set that are not in the other sets. The syntax is SDIFF key1 key2 [key3 ...], where it returns all elements in key1 that do not exist in the other specified sets.

Syntax

The SDIFF command takes one or more set keys and returns the members of the first set that are not present in any of the other sets.

• key1: The key of the first set to compare.
• key2, key3, ...: Other set keys to compare against.

The result is a set of elements unique to key1.

redis

SDIFF key1 key2 [key3 ...]

Example

This example shows how to find elements in the set fruits that are not in the set citrus.

redis

SADD fruits apple banana orange mango
SADD citrus orange lemon lime
SDIFF fruits citrus

Output

1) "apple"
2) "banana"
3) "mango"

Common Pitfalls

Common mistakes when using SDIFF include:

• Using keys that do not exist, which returns an empty set.
• Expecting SDIFF to modify sets; it only returns the difference without changing data.
• Confusing SDIFF with SDIFFSTORE, which stores the result in a new set.

redis

;; Wrong: expecting SDIFF to change sets
SDIFF fruits citrus
SMEMBERS fruits

;; Right: SDIFF only returns difference
SDIFF fruits citrus

;; To store difference use SDIFFSTORE
SDIFFSTORE unique_fruits fruits citrus
SMEMBERS unique_fruits

Output

1) "apple"
2) "banana"
3) "mango"

1) "apple"
2) "banana"
3) "orange"
4) "mango"

1) "apple"
2) "banana"
3) "mango"

Quick Reference

Command	Description
SDIFF key1 key2 [key3 ...]	Returns members of key1 not in other sets
SDIFFSTORE dest key1 key2 [key3 ...]	Stores the difference in dest set
SADD key member [member ...]	Adds members to a set
SMEMBERS key	Returns all members of a set

Key Takeaways

SDIFF returns members in the first set that are not in the other sets.

SDIFF does not modify any sets; it only returns the difference.

Use SDIFFSTORE to save the difference into a new set.

If any key does not exist, it is treated as an empty set.

Always check your keys exist to avoid unexpected empty results.