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A set is an unordered collection of unique string members. Unlike lists, a set will never hold two identical values — attempting to add a duplicate is silently ignored. Sets excel at tracking membership: unique visitors, active session IDs, feature flags enabled for an account, or any group where you need fast O(1) existence checks and don’t care about order.

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SADD

Adds one or more members to the set stored at key. Members that already belong to the set are ignored. If key does not exist, a new set is created. Syntax: SADD key member [member ...] 
SADD tags "redis" "cache" "database"
# (integer) 3

SADD tags "cache"
# (integer) 0  (already a member, nothing added)

SADD tags "search" "vector"
# (integer) 2
Returns: Integer — the number of members that were newly added (duplicates are not counted).

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SREM

Removes one or more members from the set stored at key. Members that are not in the set are silently ignored. Syntax: SREM key member [member ...] 
SADD roles "admin" "editor" "viewer"

SREM roles "editor"
# (integer) 1

SREM roles "superuser"
# (integer) 0  (member did not exist)
Returns: Integer — the number of members that were actually removed.

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SMEMBERS

Returns all members of the set stored at key. Because sets are unordered, the order of members in the response is not guaranteed. Syntax: SMEMBERS key 
SADD fruits "apple" "banana" "cherry"

SMEMBERS fruits
# 1) "banana"
# 2) "cherry"
# 3) "apple"
Returns: Array of bulk strings containing every member. Returns an empty array if the key does not exist.
The order of elements returned by SMEMBERS is not deterministic. If you need a stable order, sort the result in your application layer.

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SISMEMBER

Tests whether member belongs to the set stored at key. Syntax: SISMEMBER key member 
SADD permissions "read" "write"

SISMEMBER permissions "read"
# (integer) 1

SISMEMBER permissions "delete"
# (integer) 0
Returns: 1 if the member exists in the set, 0 if it does not (or if the key does not exist).

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SCARD

Returns the cardinality (number of members) of the set stored at key. Syntax: SCARD key 
SADD online_users "u1" "u2" "u3"

SCARD online_users
# (integer) 3

SCARD nonexistent
# (integer) 0
Returns: Integer — the number of members, or 0 if the key does not exist.

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Practical example: tracking unique visitors

Sets are a natural choice for daily unique-visitor counting. Each user ID is recorded at most once per day, and you can query the exact count at any time: 
# Record visits for January 1st
SADD visitors:2024-01-01 user1 user2 user3
# (integer) 3

# user2 visits again — duplicate is ignored
SADD visitors:2024-01-01 user2
# (integer) 0

# Check if a specific user visited
SISMEMBER visitors:2024-01-01 user2
# (integer) 1

SISMEMBER visitors:2024-01-01 user99
# (integer) 0

# Exact unique count
SCARD visitors:2024-01-01
# (integer) 3

# See all unique visitors
SMEMBERS visitors:2024-01-01
# 1) "user1"
# 2) "user2"
# 3) "user3"
Combine EXPIRE with your visitor sets to automatically clean up old day-buckets. For example: EXPIRE visitors:2024-01-01 604800 will remove the key after seven days.