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ZCOUNT

Introduction​

In Dragonfly, as well as in Redis and Valkey, the ZCOUNT command is used to count the members in a sorted set with scores within a specific range. It allows you to quickly retrieve how many elements have scores between two given values, making it particularly useful for leaderboards, ranking systems, and range-based queries.

Syntax​

ZCOUNT key min max
  • Time complexity: O(log(N)) with N being the number of elements in the sorted set.
  • ACL categories: @read, @sortedset, @fast

Parameter Explanations​

  • key: The key of the sorted set whose members will be counted.
  • min: The minimum score for the range.
  • max: The maximum score for the range.

Both min and max can be:

  • Exact numbers (e.g., 5)
  • -inf or +inf to represent negative or positive infinity, respectively
  • Inclusive or exclusive bounds, where you prepend ( to the value for exclusive comparisons (e.g., (5 excludes score 5).

Return Values​

The command returns an integer, representing the number of members in the sorted set that have scores within the specified range.

Code Examples​

Basic Example​

Count all members with a score between 1 and 4:

dragonfly$> ZADD myzset 1 "one" 2 "two" 3 "three" 4 "four"
(integer) 4
dragonfly$> ZCOUNT myzset 1 4
(integer) 4

Counting Exclusive Range​

Count members where the score is between 1 (exclusive) and 4 (inclusive):

dragonfly$> ZCOUNT myzset (1 4
(integer) 3

Using Negative and Positive Infinity​

Count members without specifying an upper or lower bound using -inf and +inf:

dragonfly$> ZCOUNT myzset -inf +inf
(integer) 4

Counting with Exclusive Upper Bound​

Count members where the score is between 1 and 4, but exclude 4:

dragonfly$> ZCOUNT myzset 1 (4
(integer) 3

Best Practices​

  • When unsure of the score range, use -inf and +inf to include all members.
  • Combine inclusive and exclusive bounds for precise member selection based on scores.
  • Consider caching common range queries if you frequently need to count elements in specific ranges.

Common Mistakes​

  • Using parentheses ( incorrectly when specifying exclusive ranges. Always prepend ( to the number for exclusivity.
  • Forgetting that min and max refer to sorted set scores, not values.
  • Assuming that ZCOUNT returns the elements themselves; it only returns the count.

FAQs​

What happens if the key does not exist?​

If the key does not exist, ZCOUNT will return 0, meaning there are no members to count.

Can I count a range where min is greater than max?​

If min is greater than max, ZCOUNT will return 0 since no members can satisfy that condition.

What’s the difference between ZCOUNT and ZRANGE?​

While ZCOUNT returns the number of elements in a score range, ZRANGE returns the elements themselves. If you only need the count, ZCOUNT is more efficient.