HDEL for field removal in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Redis command HDEL do?

The HDEL command removes one or more fields from a hash stored at a key. It deletes the specified fields and returns the number of fields that were removed.

beginner

How do you remove multiple fields from a Redis hash using HDEL?

You list all the fields you want to remove after the key. For example: HDEL myhash field1 field2 removes both field1 and field2 from the hash myhash.

intermediate

What happens if you try to remove a field that does not exist in a Redis hash using HDEL?

If the field does not exist, HDEL simply ignores it and does not count it as removed. The command returns the number of fields actually removed.

beginner

Can HDEL remove fields from a key that does not exist in Redis?

Yes. If the key does not exist, HDEL returns 0 because there are no fields to remove.

beginner

What is the return value of HDEL?

It returns an integer representing how many fields were removed from the hash.

What does the Redis command HDEL myhash field1 do?

A Deletes the entire hash <code>myhash</code>

B Removes the field <code>field1</code> from the hash <code>myhash</code>

C Adds a new field <code>field1</code> to the hash <code>myhash</code>

D Returns the value of <code>field1</code> in <code>myhash</code>

HDEL removes specified fields from a hash. Here, it removes field1 from myhash.

If you run HDEL myhash field1 field2 and only field1 exists, what is the return value?

A An error

B 0

C 2

D 1

The command returns the number of fields actually removed. Since only field1 exists, it returns 1.

What happens if you use HDEL on a key that does not exist?

A It returns 0

B It creates the key

C It returns an error

D It removes all fields

If the key does not exist, HDEL returns 0 because there are no fields to remove.

Which data type does HDEL operate on in Redis?

A Hash

B List

C Set

D String

HDEL works on hashes, which are collections of field-value pairs.

Can HDEL remove fields from multiple hashes at once?

A Yes, by listing multiple keys

B Yes, but only if the hashes have the same fields

C No, it only works on one hash key at a time

D Yes, but only in Redis Cluster mode

HDEL removes fields from a single hash key per command.

Explain how the HDEL command works in Redis and what it returns.

Think about deleting parts of a dictionary in Redis.

Describe a real-life example where you might use HDEL in a Redis hash.

Imagine a user profile stored as a hash with many fields.

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