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ZCARD

Introduction and Use Case(s)​

The ZCARD command is used in Redis to get the number of members in a sorted set. This is particularly useful when you need to determine the size of a sorted set without retrieving all its values, which can be helpful for optimizing performance or managing data structures where the count of elements matters.

Syntax​

ZCARD key

Parameter Explanations​

  • key: This is the name of the sorted set. It is a required parameter. The key must reference an existing sorted set; otherwise, the command will return 0.

Return Values​

  • Integer reply: Returns the number of elements in the sorted set. If the key does not exist, it returns 0.

Example:​

If myzset contains three elements:

dragonfly> ZCARD myzset
(integer) 3

If myzset does not exist:

dragonfly> ZCARD myzset
(integer) 0

Code Examples​

dragonfly> ZADD myzset 1 "one"
(integer) 1
dragonfly> ZADD myzset 2 "two"
(integer) 1
dragonfly> ZADD myzset 3 "three"
(integer) 1
dragonfly> ZCARD myzset
(integer) 3
dragonfly> DEL myzset
(integer) 1
dragonfly> ZCARD myzset
(integer) 0

Best Practices​

  • Memory Efficiency: Use ZCARD to check the number of elements before performing operations that depend on the size of the sorted set.

Common Mistakes​

  • Key Not Existing: Assuming that ZCARD will return an error if the key does not exist. It actually returns 0.
  • Wrong Data Type: Using ZCARD on keys that are not sorted sets. This will result in an error.

FAQs​

What happens if I use ZCARD on a non-sorted set key?​

You will receive an error indicating that the key holds the wrong kind of value because ZCARD only works with sorted sets.

Can ZCARD be used to check the existence of a sorted set?​

Yes, indirectly. If ZCARD returns 0, the sorted set either does not exist or is empty. To explicitly check for existence, you might prefer using the EXISTS command.