Skip to main content

ZREM

Introduction​

In Dragonfly, as well as in Redis and Valkey, the ZREM command is used to remove one or more members from a sorted set stored at a specified key. This can be useful when managing priority queues, leaderboards, or any scenario where members of a set need to be dynamically removed while maintaining ordering based on score.

Syntax​

ZREM key member [member ...]
  • Time complexity: O(M*log(N)) with N being the number of elements in the sorted set and M the number of elements to be removed.
  • ACL categories: @write, @sortedset, @fast

Parameter Explanations​

  • key: The sorted set from which members are to be removed.
  • member: One or more members to remove from the sorted set. Members that do not exist are ignored.

Return Values​

  • The command returns an integer representing the number of members that were removed from the sorted set.

Code Examples​

Basic Example​

Remove a single member from a sorted set:

dragonfly$> ZADD myzset 1 "alpha" 2 "beta" 3 "gamma"
(integer) 3

dragonfly$> ZREM myzset "alpha"
(integer) 1

dragonfly$> ZRANGE myzset 0 -1
1) "beta"
2) "gamma"

Remove Multiple Members​

You can also remove multiple members in one command:

dragonfly$> ZADD myzset 1 "alpha" 2 "beta" 3 "gamma"
(integer) 3

dragonfly$> ZREM myzset "alpha" "gamma"
(integer) 2

dragonfly$> ZRANGE myzset 0 -1
1) "beta"

Attempt to Remove Non-Existing Members​

If you attempt to remove members that do not exist in the sorted set, they are simply ignored:

dragonfly$> ZADD myzset 1 "alpha" 2 "beta"
(integer) 2

dragonfly$> ZREM myzset "gamma" "delta"
(integer) 0  # No members were removed because "gamma" and "delta" don't exist.

Best Practices​

  • Use ZREM in conjunction with other sorted set commands like ZRANGE and ZADD to maintain leaderboards, priority queues, or other ranked data structures.
  • When removing members, consider batching multiple deletions in a single ZREM command to minimize round-trip time and improve performance.

Common Mistakes​

  • Attempting to remove members from a key that is not a sorted set will result in an error.
  • Expecting ZREM to return an error when attempting to remove non-existent members; instead, it simply returns the count of successfully removed members.

FAQs​

What happens if the key does not exist?​

If the key does not exist, ZREM will return 0 since there are no members to remove.

Can I use ZREM with an empty member list?​

No, attempting to run ZREM without specifying at least one member will result in an error.