Redis provides three commands to count the items within a sorted set: ZCARD, ZCOUNT, ZLEXCOUNT.
The ZCARD command is the basic test for the cardinality of a set. (It is analogous to the SCARD command for sets.) . ZCARD returns the count of the members of a set. Executing the following code to add items to a set:
zadd favs 1 apple zadd favs 2 pizza zadd favs 3 chocolate zadd favs 4 beer
returns a value of 4.
The ZCOUNT and ZLEXCOUNT commands allow you to count a subset of the items in a sorted set based on a range of values. ZCOUNT allows you to count items within a particular range of scores and ZLEXCOUNT allowes you to count the number of items within a particular lexographic range.
Using our set above:
zcount favs 2 5
would return a 3, since there are three items (pizza, chocolate, beer) that have scores between 2 and 5 inclusive.
ZLEXCOUNT is designed to work with sets where every item has the same score, forcing and ordering on the elemement names. If we created a set like:
zadd favs 1 apple zadd favs 1 pizza zadd favs 1 chocolate zadd favs 1 beer
we could use ZLEXCOUNT to get the number of elements in particular lexographical range (this is done by byte-wise comparison using the memcpy function).
zlexcount favs [apple (chocolate
would return 2, since two elements (apple, beer) fall within the range apple (inclusive) and chocolate (exclusive). We could alternatively make both ends inclusive:
zlexcount favs [apple [chocolate
and get the result 3.