Sorted Sets

What is a Redis Sorted Set?

A Sorted Set is like a Set but each member carries a floating-point score. Redis keeps the members sorted by score at all times, with O(log N) insertions and range queries. If two members share the same score, they are sorted lexicographically.

ZADD — adding members with scores

ZADD key score member [score member ...] adds members. If a member already exists its score is updated. Returns the number of new members added.

127.0.0.1:6379> ZADD game:scores 4200 "alice" 3800 "bob" 5100 "carol"
(integer) 3
127.0.0.1:6379> ZADD game:scores 4700 "alice"
(integer) 0
127.0.0.1:6379> ZSCORE game:scores "alice"
"4700"

ZADD game:scores 4200 "alice" 3800 "bob" 5100 "carol"
ZADD game:scores 4700 "alice"
ZSCORE game:scores "alice"

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ZRANGE and ZREVRANGE — range by rank

ZRANGE key start stop [WITHSCORES] returns members in ascending score order (lowest first). ZREVRANGE returns them in descending order (highest first). Indices are 0-based; -1 means the last member.

127.0.0.1:6379> ZADD leaderboard 900 "carol" 1500 "alice" 2100 "bob" 750 "dave"
(integer) 4
127.0.0.1:6379> ZRANGE leaderboard 0 -1 WITHSCORES
1) "dave"
2) "750"
3) "carol"
4) "900"
5) "alice"
6) "1500"
7) "bob"
8) "2100"
127.0.0.1:6379> ZREVRANGE leaderboard 0 2 WITHSCORES
1) "bob"
2) "2100"
3) "alice"
4) "1500"
5) "carol"
6) "900"

ZADD leaderboard 900 "carol" 1500 "alice" 2100 "bob" 750 "dave"
ZRANGE leaderboard 0 -1 WITHSCORES
ZREVRANGE leaderboard 0 2 WITHSCORES

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ZRANK and ZSCORE — finding a member

ZRANK key member returns the 0-based rank of a member in ascending order. ZREVRANK gives rank in descending order. ZSCORE returns the score as a bulk string.

127.0.0.1:6379> ZADD players 300 "x" 600 "y" 900 "z"
(integer) 3
127.0.0.1:6379> ZRANK players "y"
(integer) 1
127.0.0.1:6379> ZREVRANK players "y"
(integer) 1
127.0.0.1:6379> ZSCORE players "z"
"900"

ZADD players 300 "x" 600 "y" 900 "z"
ZRANK players "y"
ZREVRANK players "y"
ZSCORE players "z"

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ZRANGEBYSCORE — range by score value

ZRANGEBYSCORE key min max [WITHSCORES] fetches all members whose score falls between min and max (inclusive). Use -inf and +inf for open bounds.

127.0.0.1:6379> ZADD temps 18.5 "london" 32.1 "bangkok" 24.0 "berlin" 28.7 "tokyo"
(integer) 4
127.0.0.1:6379> ZRANGEBYSCORE temps 20 30 WITHSCORES
1) "berlin"
2) "24"
3) "tokyo"
4) "28.7"

ZADD temps 18.5 "london" 32.1 "bangkok" 24.0 "berlin" 28.7 "tokyo"
ZRANGEBYSCORE temps 20 30 WITHSCORES

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ZINCRBY, ZCARD, and ZREM

ZINCRBY key increment member atomically adds increment to a member’s score. ZCARD key returns the total number of members. ZREM key member [member ...] removes one or more members.

127.0.0.1:6379> ZADD votes 10 "post:1" 5 "post:2" 8 "post:3"
(integer) 3
127.0.0.1:6379> ZINCRBY votes 3 "post:2"
"8"
127.0.0.1:6379> ZCARD votes
(integer) 3
127.0.0.1:6379> ZREM votes "post:3"
(integer) 1
127.0.0.1:6379> ZCARD votes
(integer) 2

ZADD votes 10 "post:1" 5 "post:2" 8 "post:3"
ZINCRBY votes 3 "post:2"
ZCARD votes
ZREM votes "post:3"
ZCARD votes

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🟥 Members are unique — adding an existing member updates its score

A Sorted Set will never contain the same member twice. When you ZADD a member that is already present, Redis updates its score and returns 0 (no new members added). This makes ZADD a safe upsert for counters, ratings, and timestamps — no need to check whether the member exists first.

What does ZREVRANGE return compared to ZRANGE?

Members in insertion order Members in descending score order Members in ascending score order Only the top member

What does ZRANK return for a member?

Its score Its 0-based position in ascending score order The total number of members Its 1-based position

What happens when you ZADD a score for a member that already exists?

An error is returned A duplicate is created The score is updated and 0 is returned The old score is kept

Which command atomically adds a value to an existing member's score?

ZADD ZSCORE ZINCRBY ZRANGEBYSCORE

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