ZCARD for set size in Redis - Step-by-Step Execution

Concept Flow - ZCARD for set size

Start

↓

Call ZCARD with key

↓

Check if key exists

↓

Count elements

↓

Return count

The ZCARD command checks if the sorted set key exists, counts its elements if yes, or returns 0 if no.

Execution Sample

Redis

ZADD myset 1 one 2 two 3 three
ZCARD myset
ZCARD unknownset

Add three elements to a sorted set 'myset', then get its size with ZCARD, and check size of a non-existing set.

Execution Table

Step	Command	Key	Key Exists?	Action	Output
1	ZADD	myset	No	Create 'myset' and add 3 elements	3
2	ZCARD	myset	Yes	Count elements in 'myset'	3
3	ZCARD	unknownset	No	Return 0 because key does not exist	0

💡 Execution stops after returning the count or 0 for each ZCARD command.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
myset	does not exist	sorted set with 3 elements	unchanged	unchanged
unknownset	does not exist	does not exist	does not exist	does not exist

Key Moments - 2 Insights

Why does ZCARD return 0 for a key that does not exist?

Because as shown in execution_table row 3, when the key is missing, Redis returns 0 instead of an error.

Does ZCARD count the number of elements or sum their scores?

ZCARD counts the number of elements only, not their scores, as shown in row 2 where output is 3 for three elements.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output of ZCARD for 'myset' at step 2?

A Error

B 0

C 3

D 1

💡 Hint

Check the 'Output' column in execution_table row 2.

At which step does Redis create the sorted set 'myset'?

A Step 3

B Step 1

C Step 2

D No creation happens

💡 Hint

Look at the 'Action' column in execution_table row 1.

If 'unknownset' existed with 5 elements, what would ZCARD return at step 3?

A 5

B 0

C Error

D 1

💡 Hint

ZCARD returns the count of elements if the key exists, see execution_table row 2 for example.

Concept Snapshot

ZCARD key
- Returns the number of elements in a sorted set.
- If key does not exist, returns 0.
- Does not sum scores, only counts elements.
- Useful to check size of sorted sets quickly.

Full Transcript

The ZCARD command in Redis returns the number of elements in a sorted set stored at the given key. If the key does not exist, it returns 0 instead of an error. For example, after adding three elements to 'myset', ZCARD 'myset' returns 3. If you call ZCARD on a non-existing key like 'unknownset', it returns 0. This command helps you quickly find out how many members are in a sorted set without retrieving the elements themselves.