HEXISTS for field check in Redis - Step-by-Step Execution

Concept Flow - HEXISTS for field check

Start

↓

Call HEXISTS key field

↓

Check if key exists?

No→Return 0 (field not found)

Yes↓

Check if field exists in hash

↓

Return 1 (field exists)

↓

Return 0 (field not found)

HEXISTS checks if a specific field exists inside a hash stored at a key. It returns 1 if found, otherwise 0.

Execution Sample

Redis

HSET user:1 name "Alice"
HSET user:1 age 30
HEXISTS user:1 name
HEXISTS user:1 email

Set fields 'name' and 'age' in hash 'user:1', then check if 'name' and 'email' fields exist.

Execution Table

Step	Command	Key Exists?	Field Exists?	Return Value
1	HSET user:1 name "Alice"	No -> Yes (created)	name added	1 (field added)
2	HSET user:1 age 30	Yes	age added	1 (field added)
3	HEXISTS user:1 name	Yes	Yes	1
4	HEXISTS user:1 email	Yes	No	0

💡 HEXISTS returns 1 if field exists in hash, else 0; key must exist for field check

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
user:1 hash fields	empty	{"name": "Alice"}	{"name": "Alice", "age": "30"}	{"name": "Alice", "age": "30"}	{"name": "Alice", "age": "30"}

Key Moments - 2 Insights

Why does HEXISTS return 0 for 'email' even though the key 'user:1' exists?

HEXISTS checks if the specific field exists inside the hash. In step 4, 'email' field is not present in 'user:1', so it returns 0 (see execution_table row 4).

What happens if the key does not exist at all when HEXISTS is called?

If the key does not exist, HEXISTS immediately returns 0 because there is no hash to check fields in (see concept_flow).

Visual Quiz - 3 Questions

Look at the execution_table, what does HEXISTS return at step 3 for field 'name'?

A 1

B null

C 0

D error

💡 Hint

Check the 'Return Value' column in execution_table row 3

At which step does the key 'user:1' get created?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at 'Key Exists?' column in execution_table row 1

If we check HEXISTS for a field on a non-existent key, what would be the return value?

A 1

B 0

C null

D error

💡 Hint

Refer to concept_flow where key existence is checked first

Concept Snapshot

HEXISTS key field
- Checks if 'field' exists in hash at 'key'
- Returns 1 if field exists, 0 otherwise
- Returns 0 if key does not exist
- Useful to test presence of a field before reading or updating

Full Transcript

HEXISTS is a Redis command to check if a specific field exists inside a hash stored at a given key. When you run HEXISTS with a key and a field, Redis first checks if the key exists. If the key does not exist, it returns 0 immediately. If the key exists, it then checks if the field exists inside the hash. If the field is found, it returns 1; if not, it returns 0. For example, if you set fields 'name' and 'age' in a hash called 'user:1', then HEXISTS user:1 name returns 1 because 'name' exists, but HEXISTS user:1 email returns 0 because 'email' was never set. This command helps you know if a field is present before trying to read or modify it.