HDEL for field removal in Redis - Step-by-Step Execution

Concept Flow - HDEL for field removal

Start with a hash key

↓

Check if field exists in hash

Yes↓

Remove the field

↓

Return number of fields removed

↓

End

HDEL removes specified fields from a hash if they exist and returns how many fields were removed.

Execution Sample

Redis

HSET user:1 name "Alice" age "30" city "NY"
HDEL user:1 age city
HGETALL user:1

This code sets fields in a hash, removes two fields, then retrieves remaining fields.

Execution Table

Step	Command	Action	Result	Hash State
1	HSET user:1 name "Alice" age "30" city "NY"	Create hash with fields	3 (fields added)	{"name":"Alice", "age":"30", "city":"NY"}
2	HDEL user:1 age city	Remove fields 'age' and 'city'	2 (fields removed)	{"name":"Alice"}
3	HGETALL user:1	Get all fields and values	["name", "Alice"]	{"name":"Alice"}
4	HDEL user:1 country	Try to remove non-existing field	0 (no fields removed)	{"name":"Alice"}

💡 No more commands; hash now only has 'name' field.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
user:1 hash	{}	{"name":"Alice", "age":"30", "city":"NY"}	{"name":"Alice"}	{"name":"Alice"}	{"name":"Alice"}
HDEL result	N/A	N/A	2	N/A	0

Key Moments - 2 Insights

What happens if you try to remove a field that does not exist?

The HDEL command returns 0 and the hash remains unchanged, as shown in step 4 of the execution_table.

Does HDEL remove multiple fields at once?

Yes, HDEL can remove multiple fields in one command, removing only those that exist, as shown in step 2.

Visual Quiz - 3 Questions

Look at the execution_table, what is the hash state after step 2?

A {}

B {"age":"30", "city":"NY"}

C {"name":"Alice"}

D {"name":"Alice", "age":"30", "city":"NY"}

💡 Hint

Check the 'Hash State' column in row for step 2.

At which step does HDEL return 0 because no fields were removed?

A Step 4

B Step 1

C Step 3

D Step 2

💡 Hint

Look at the 'Result' column for each step in the execution_table.

If you add a new field 'country' before step 4, what would HDEL return when removing 'country'?

A 0

B 1

C 2

D Error

💡 Hint

HDEL returns the number of fields actually removed; see step 2 for multiple removals.

Concept Snapshot

HDEL key field [field ...]
Removes specified fields from a hash.
Returns number of fields removed.
If field doesn't exist, it's ignored.
Useful to clean up hash entries.

Full Transcript

The HDEL command in Redis removes one or more specified fields from a hash stored at a given key. If the fields exist, they are deleted and the command returns how many fields were removed. If a field does not exist, it is ignored and does not affect the result. For example, after setting fields 'name', 'age', and 'city' in a hash, running HDEL on 'age' and 'city' removes those two fields, leaving only 'name'. Trying to remove a non-existing field returns zero and leaves the hash unchanged. This command helps manage hash data by removing unwanted fields efficiently.

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