Sets & Sorted Sets: Overview

Two types, one concept: uniqueness

Redis gives you two collection types that guarantee every member appears at most once:

Type	Order	Extra data	Best for
Set	None (hash-table)	None	Tags, membership checks, set algebra
Sorted Set	By score (float64)	Score per member	Leaderboards, ranked feeds, range queries

Both types store unique strings. The difference is that a Sorted Set attaches a floating-point score to each member, and keeps them sorted by that score at all times.

Sets at a glance

A Set is an unordered collection of unique strings. Adding the same value twice has no effect.

127.0.0.1:6379> SADD lang:tags "python" "go" "rust" "go"
(integer) 3
127.0.0.1:6379> SMEMBERS lang:tags
1) "rust"
2) "go"
3) "python"
127.0.0.1:6379> SCARD lang:tags
(integer) 3

SADD lang:tags "python" "go" "rust" "go"
SMEMBERS lang:tags
SCARD lang:tags

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Sorted Sets at a glance

A Sorted Set pairs each member with a score. Members are always retrieved in score order.

127.0.0.1:6379> ZADD scores 1500 "alice" 2200 "bob" 980 "carol"
(integer) 3
127.0.0.1:6379> ZRANGE scores 0 -1 WITHSCORES
1) "carol"
2) "980"
3) "alice"
4) "1500"
5) "bob"
6) "2200"
127.0.0.1:6379> ZRANK scores "alice"
(integer) 1

ZADD scores 1500 "alice" 2200 "bob" 980 "carol"
ZRANGE scores 0 -1 WITHSCORES
ZRANK scores "alice"

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When to use each

• Set — you need to answer “is X a member?” or “what do A and B have in common?” quickly.
• Sorted Set — you need ordering, ranking, or range queries (e.g., “players with scores between 1000 and 2000”).

🟥 Uniqueness is guaranteed at write time

Both Sets and Sorted Sets enforce uniqueness automatically. You never need to check before inserting — the data structure does it for you. For Sorted Sets, adding a member that already exists simply updates its score.

What this module covers

Lesson	Topic
Sets & Sorted Sets: Overview	This page — types and a quick taste
Sets	SADD, SREM, SMEMBERS, SISMEMBER, SCARD, SRANDMEMBER, SPOP
Set Operations	SINTER, SUNION, SDIFF and their STORE variants
Sorted Sets	ZADD, ZSCORE, ZRANGE, ZREVRANGE, ZRANK, ZRANGEBYSCORE, ZINCRBY, ZCARD, ZREM
Leaderboards	Building a real leaderboard with Sorted Sets

What guarantee do both Sets and Sorted Sets share?

All members have a score Members are stored in insertion order Every member is unique Members expire automatically

What extra piece of data does a Sorted Set store per member?

A rank integer A float64 score A TTL A binary payload

What happens when you ZADD a member that already exists in a Sorted Set?

An error is returned A duplicate is created Its score is updated The command is ignored

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