EXISTS to check key existence in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Check if a single key exists

Given the Redis database contains keys: user:1, user:2, and session:1, what is the output of the command EXISTS user:1?

Redis

EXISTS user:1

A 1

B 0

C Error: key not found

D null

💡 Hint

EXISTS returns 1 if the key exists, otherwise 0.

The EXISTS command returns 1 if the key is present in the database. Since user:1 exists, the output is 1.

❓ query_result

intermediate

Check multiple keys existence count

What does the command EXISTS user:1 user:3 session:1 return if only user:1 and session:1 exist in the Redis database?

Redis

EXISTS user:1 user:3 session:1

A 3

B 1

C 2

D 0

💡 Hint

EXISTS returns the number of keys that exist among the given keys.

Only user:1 and session:1 exist, so EXISTS returns 2.

📝 Syntax

advanced

Identify the syntax error in EXISTS command

Which option shows an invalid syntax for the EXISTS command in Redis?

A EXISTS user:3

B EXISTS (user:1)

C EXISTS session:1

D EXISTS user:1 user:2

💡 Hint

EXISTS command does not use parentheses around keys.

Parentheses are not valid syntax in Redis commands. Option B uses parentheses, causing a syntax error.

❓ optimization

advanced

Optimizing key existence checks

You want to check if any of the keys user:1, user:2, user:3 exist. Which command is the most efficient to get the count of existing keys?

A EXISTS user:1; EXISTS user:2; EXISTS user:3

B KEYS user:*

C GET user:1 user:2 user:3

D EXISTS user:1 user:2 user:3

💡 Hint

EXISTS can take multiple keys at once and returns the count of existing keys.

Using EXISTS with multiple keys in one command is efficient and returns the count directly. Option D sends multiple commands, which is less efficient.

🧠 Conceptual

expert

Behavior of EXISTS with expired keys

If a key named session:temp has expired and been deleted by Redis, what will EXISTS session:temp return?

Redis

EXISTS session:temp

A 0

B 1

C Error: key expired

D null

💡 Hint

Expired keys are automatically removed and do not exist anymore.

Once a key expires, it is deleted and no longer exists. EXISTS returns 0 for such keys.