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.