SCARD for set size in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of SCARD on a non-empty set?

Given a Redis set stored at key myset containing the elements {"apple", "banana", "cherry"}, what will the command SCARD myset return?

Redis

SADD myset apple banana cherry
SCARD myset

A 3

B 1

C Error: key does not exist

D 0

💡 Hint

SCARD returns the number of elements in the set stored at the given key.

The SCARD command returns the number of elements in the set stored at the specified key. Since myset contains three elements, the output is 3.

❓ query_result

intermediate

What does SCARD return for a non-existent key?

If you run SCARD unknownkey on a Redis database where unknownkey does not exist, what will be the output?

Redis

SCARD unknownkey

A Error: key does not exist

B 0

C null

D -1

💡 Hint

SCARD returns zero if the key does not exist or is empty.

When the key does not exist, Redis treats it as an empty set. Therefore, SCARD returns 0.

📝 Syntax

advanced

Which SCARD command syntax is correct?

Identify the correct syntax to get the size of a set named fruits in Redis.

A SCARD:fruits

B SCARD(fruits)

C SCARD fruits

D SCARD = fruits

💡 Hint

Redis commands use space-separated arguments without parentheses or symbols.

The correct syntax for the SCARD command is SCARD key. Parentheses, colons, or equal signs are not used.

❓ optimization

advanced

Efficiently checking if a Redis set is empty

You want to check if the Redis set users_online is empty without retrieving all members. Which command is the most efficient?

A SCARD users_online

B SMEMBERS users_online

C SISMEMBER users_online ""

D GET users_online

💡 Hint

Think about which command returns only the count without fetching all data.

SCARD returns the number of elements in the set efficiently. SMEMBERS returns all members, which is costly. SISMEMBER checks for a specific member, and GET is for strings, not sets.

🧠 Conceptual

expert

Understanding SCARD behavior with different data types

What happens if you run SCARD on a Redis key that holds a string value instead of a set?

Redis

SET mykey "hello"
SCARD mykey

A 0

B null

C 1

D Error: WRONGTYPE Operation against a key holding the wrong kind of value

💡 Hint

Redis commands are type-specific and will error if used on wrong data types.

If SCARD is run on a key holding a string, Redis returns a WRONGTYPE error because the command expects a set.