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ZSCORE

Introduction​

In Dragonfly, as well as in Redis and Valkey, the ZSCORE command is used to get the score associated with a member in a sorted set. Each element in a sorted set is ordered according to its score, so this command is especially helpful to query and retrieve a specific element's score efficiently.

Syntax​

ZSCORE key member
  • Time complexity: O(1)
  • ACL categories: @read, @sortedset, @fast

Parameter Explanations​

  • key: The key of the sorted set where the member belongs.
  • member: The member whose score you want to retrieve.

Return Values​

  • The command returns the score (as bulk string in RESP2 or as double in RESP3) associated with the specified member.
  • If the member does not exist within the sorted set, nil is returned.

Code Examples​

Basic Example​

Retrieve the score of a specific member from the sorted set:

dragonfly$> ZADD myzset 1 "member1" 2 "member2" 3 "member3"
(integer) 3

dragonfly$> ZSCORE myzset "member2"
"2"

Member Not Found​

If a member is not present in a sorted set, ZSCORE will return nil:

dragonfly$> ZADD myzset 1 "member1" 2 "member2" 3 "member3"
(integer) 3

dragonfly$> ZSCORE myzset "non_existent_member"
(nil)

Score for Members with Decimal Values​

Sorted sets can store floating-point scores, and ZSCORE can retrieve them as well:

dragonfly$> ZADD myzset 1.5 "memberA" 3.7 "memberB"
(integer) 2

dragonfly$> ZSCORE myzset "memberB"
"3.7"

Best Practices​

  • Use the ZSCORE command when you only need to retrieve the score of a single member as it provides a direct and efficient method to perform this query.
  • When working with large sorted sets, ensure that looks up for non-existent members won't clutter your application's logic; handle the nil return value accordingly.
  • To get multiple scores at once, consider pairing ZSCORE with other sorted set operations like ZRANGE or ZSCAN for more complex queries. Pipelining multiple ZSCORE commands can help reduce the number of round trips to the server.

Common Mistakes​

  • Expecting ZSCORE to return a numeric value. The score is returned as a string in RESP2, and you'll need to convert it if further numeric operations are needed. However, established client libraries should handle the returned value properly.
  • Assuming that a negative score means the member is ranked lower in the sorted set. The sorted set will order by score, and not necessarily by logic derived from the score's value.

FAQs​

What happens if the key does not exist?​

If the key does not exist, ZSCORE returns nil.

Can I use ZSCORE with non-numeric strings as members?​

Yes, members in sorted sets are just strings, so they can be any valid string value. However, the scores associated with them must be numeric.