HDEL for field removal in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result of this HDEL command?

Given a Redis hash stored as user:1000 with fields name, email, and age, what does the command HDEL user:1000 email return?

Redis

HDEL user:1000 email

A The entire hash is deleted

B 0

C An error because the field does not exist

D 1

💡 Hint

HDEL returns the number of fields it successfully removed.

HDEL returns 1 if the specified field existed and was removed, otherwise 0.

❓ query_result

intermediate

What happens if you try to remove a non-existing field?

If the Redis hash session:123 has fields token and expiry, what does HDEL session:123 user return?

Redis

HDEL session:123 user

A 0

B 1

C An error because the field does not exist

D Deletes the entire hash

💡 Hint

HDEL returns how many fields it removed, zero if none.

Since 'user' field does not exist, HDEL returns 0.

📝 Syntax

advanced

Which HDEL command syntax is correct to remove multiple fields?

You want to remove fields name and email from the hash user:2000. Which command is valid?

A HDEL user:2000 {name,email}

B HDEL user:2000 [name, email]

C HDEL user:2000 name email

D HDEL user:2000 'name,email'

💡 Hint

HDEL accepts multiple fields as separate arguments.

The correct syntax lists fields separated by spaces without brackets or quotes.

❓ optimization

advanced

How to efficiently remove multiple fields from a large hash?

You have a large Redis hash config:settings with many fields. You want to remove 100 specific fields. Which approach is best?

A Call HDEL once with all 100 fields listed

B Call HDEL 100 times, each with one field

C Delete the entire hash and recreate it without those fields

D Use HDEL with a pattern to remove fields

💡 Hint

HDEL accepts multiple fields in one call, reducing network overhead.

Calling HDEL once with all fields is efficient and reduces round-trips.

🧠 Conceptual

expert

What is the effect of HDEL on a hash when all fields are removed?

If you remove all fields from a Redis hash using HDEL, what happens to the hash key itself?

A The hash key remains but is empty

B The hash key is automatically deleted from Redis

C The hash key becomes a string with value ''

D Redis throws an error when all fields are removed

💡 Hint

Empty hashes are automatically deleted by Redis.

Redis automatically deletes the hash key when the last field is removed (EXISTS returns 0).