How to Use SUNION Command in Redis for Set Union

Use the SUNION command in Redis to get the union of multiple sets, which returns all unique members from the given sets. The syntax is SUNION key1 key2 ..., where each key is a set name.

Syntax

The SUNION command combines all members from the specified sets and returns a list of unique elements.

• key1, key2, ...: Names of the sets to union.
• The result contains all unique members from these sets.

redis

SUNION key1 key2 [key3 ...]

Example

This example shows how to create two sets and use SUNION to get all unique members from both.

redis

SADD fruits apple banana orange
SADD tropical mango banana pineapple
SUNION fruits tropical

Output

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

Common Pitfalls

Common mistakes include:

• Using SUNION on keys that are not sets, which causes errors.
• Expecting SUNION to modify sets; it only returns the union without changing data.
• Confusing SUNION with SUNIONSTORE, which stores the result in a new set.

redis

/* Wrong: Using SUNION on a string key */
SET key1 "value"
SUNION key1 fruits

/* Right: Use only set keys */
SADD key1 a b c
SADD key2 b c d
SUNION key1 key2

Output

(error) WRONGTYPE Operation against a key holding the wrong kind of value
1) "a"
2) "b"
3) "c"
4) "d"

Quick Reference

Command	Description
SUNION key1 key2 ...	Returns the union of all given sets
SUNIONSTORE dest key1 key2 ...	Stores the union of sets into destination key
SADD key member [member ...]	Adds members to a set
SMEMBERS key	Returns all members of a set

Key Takeaways

SUNION returns all unique members from multiple sets without modifying them.

Only use SUNION with keys that hold sets to avoid errors.

SUNION does not store results; use SUNIONSTORE to save the union.

The command syntax is simple: SUNION followed by set keys.

Use SADD to create sets before using SUNION.