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A Redis set is a collection of unique elements stored under a single key.

Unlike lists, sets have two important properties:

• Elements are unique (duplicates are not allowed).
• Elements are not ordered.

Example conceptually:

online_users → {"user1", "user2", "user3"}

If you attempt to add the same element again, Redis simply ignores it.

Because of this property, sets are useful for problems where uniqueness matters.

Common uses include:

• tracking unique visitors
• storing tags
• membership checks
• relationships between objects

Adding Elements

SADD

Adds one or more elements to a set.

SADD users "alice"
SADD users "bob"

If "alice" is added again:

SADD users "alice"

The set remains unchanged because duplicates are not allowed.

Viewing Elements

SMEMBERS

Returns all members of a set.

SMEMBERS users

Output might look like:

1) "alice"
2) "bob"

The order is not guaranteed because sets are unordered.

Checking Membership

SISMEMBER

Checks whether a value exists in a set.

SISMEMBER users "alice"

Output:

(integer) 1

If the element does not exist:

(integer) 0

This operation is extremely fast and is often used for permission checks or membership validation.

Removing Elements

SREM

Removes an element from the set.

SREM users "alice"

After removal:

SMEMBERS users

Result:

"bob"

Set Operations

Redis can also perform mathematical set operations.

SINTER (Intersection)

Returns elements present in both sets.

SADD group1 "alice" "bob"
SADD group2 "bob" "charlie"

SINTER group1 group2

Result:

"bob"

SUNION (Union)

Returns elements from both sets combined.

SUNION group1 group2

Result:

"alice"
"bob"
"charlie"

SDIFF (Difference)

Returns elements that exist in one set but not the other.

SDIFF group1 group2

Result:

"alice"

Example Use Case

Tracking online users.

When a user logs in:

SADD online_users user123

When the user logs out:

SREM online_users user123

Checking if a user is online:

SISMEMBER online_users user123

This approach is fast, simple, and avoids duplicate entries.

Related: [[Redis]] [[Redis - Strings]] [[Redis - Lists]] [[Redis - Hashes]] [[Redis-sorted-sets]