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).

Practice

1. What does the Redis command HDEL do?

easy

A. Renames a hash key

B. Deletes specified fields from a hash

C. Retrieves all fields from a hash

D. Adds new fields to a hash

Solution

1. Step 1: Understand the purpose of HDEL

   The HDEL command is used to remove one or more fields from a Redis hash.

2. Step 2: Compare with other commands

   Other commands like HSET add fields, HGETALL retrieves fields, and renaming keys is done by RENAME.

3. Final Answer:

   Deletes specified fields from a hash -> Option B

4. Quick Check:

   HDEL removes fields = D [OK]

Hint: HDEL always removes fields from a hash [OK]

Common Mistakes:

• Confusing HDEL with HSET (adding fields)
• Thinking HDEL renames keys
• Assuming HDEL retrieves data

2. Which of the following is the correct syntax to remove the field age from a hash named user:1?

easy

A. HDEL age user:1

B. HREMOVE user:1 age

C. DEL user:1 age

D. HDEL user:1 age

Solution

1. Step 1: Recall HDEL syntax

   The correct syntax is HDEL key field [field ...], so the key comes first, then the field(s).

2. Step 2: Check each option

   HDEL user:1 age matches the syntax. HDEL age user:1 reverses key and field. DEL user:1 age uses DEL which deletes keys, not fields. HREMOVE user:1 age uses a non-existent command.

3. Final Answer:

   HDEL user:1 age -> Option D

4. Quick Check:

   HDEL key field = B [OK]

Hint: HDEL syntax: key first, then field(s) [OK]

Common Mistakes:

• Swapping key and field order
• Using DEL instead of HDEL
• Using wrong command names

3. Given the hash user:2 with fields {"name":"Alice", "age":"30", "city":"NY"}, what will be the result of HDEL user:2 age city followed by HGETALL user:2?

medium

A. []

B. ["age", "30", "city", "NY"]

C. ["name", "Alice"]

D. ["name", "Alice", "age", "30"]

Solution

1. Step 1: Understand HDEL effect

   The command HDEL user:2 age city removes the fields age and city from the hash.

2. Step 2: Check remaining fields

   After removal, only the field name with value Alice remains. So HGETALL user:2 returns ["name", "Alice"].

3. Final Answer:

   ["name", "Alice"] -> Option C

4. Quick Check:

   Removed age and city, only name left = A [OK]

Hint: HDEL removes listed fields, others stay [OK]

Common Mistakes:

• Expecting all fields to be deleted
• Confusing HDEL with DEL (which deletes whole key)
• Misreading output format

4. You run the command HDEL user:3 name age but get an error. Which of the following is the most likely cause?

medium

A. The key user:3 holds the wrong kind of value

B. You cannot delete multiple fields at once

C. The command should be HREMOVE instead of HDEL

D. Field names must be numeric

Solution

1. Step 1: Check command validity

   HDEL supports deleting multiple fields and is the correct command; HREMOVE does not exist.

2. Step 2: Consider error causes

   HDEL returns (integer) 0 if the key does not exist (no error). It errors (WRONGTYPE) if the key exists but is not a hash.

3. Step 3: Most common cause

   The most likely cause of an error is that the key user:3 holds the wrong kind of value.

4. Final Answer:

   The key user:3 holds the wrong kind of value -> Option A

5. Quick Check:

   Wrong type causes error = A [OK]

Hint: Check key type (TYPE key) before HDEL [OK]

Common Mistakes:

• Thinking HDEL can't delete multiple fields
• Using wrong command name
• Assuming fields must be numbers

5. You have a Redis hash settings with fields {"theme":"dark", "font":"Arial", "notifications":"on", "volume":"75"}. You want to remove all fields except theme and notifications. Which command(s) should you run?

hard

A. HDEL settings font volume

B. HDEL settings theme notifications

C. DEL settings; HSET settings theme dark notifications on

D. HDEL settings font volume theme notifications

Solution

1. Step 1: Identify fields to remove

   You want to keep theme and notifications, so remove font and volume.

2. Step 2: Choose correct HDEL usage

   HDEL settings font volume deletes only font and volume, preserving the others. HDEL settings theme notifications deletes the fields you want to keep. DEL settings; HSET settings theme dark notifications on deletes the whole hash and recreates it, which is more complex. HDEL settings font volume theme notifications tries to delete all fields including those to keep.

3. Final Answer:

   HDEL settings font volume -> Option A

4. Quick Check:

   Remove unwanted fields only = C [OK]

Hint: HDEL only fields you want removed, keep others [OK]

Common Mistakes:

• Deleting fields to keep
• Deleting entire hash unnecessarily
• Using wrong command to remove fields