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.

Practice

1. What does the Redis command HEXISTS key field do?

easy

A. Deletes the specified field from the hash

B. Sets a new field in the hash

C. Returns all fields and values in the hash

D. Checks if the specified field exists in the hash stored at key

Solution

1. Step 1: Understand the HEXISTS command purpose

   HEXISTS checks if a specific field exists inside a Redis hash stored at a given key.

2. Step 2: Identify the correct behavior

   It returns 1 if the field exists, otherwise 0. It does not delete, list, or set fields.

3. Final Answer:

   Checks if the specified field exists in the hash stored at key -> Option D

4. Quick Check:

   HEXISTS checks field existence = B [OK]

Hint: HEXISTS returns 1 if field exists, else 0 [OK]

Common Mistakes:

• Confusing HEXISTS with HDEL (delete)
• Thinking HEXISTS returns the field value
• Assuming HEXISTS lists all fields

2. Which of the following is the correct syntax to check if field age exists in hash user:1 using HEXISTS?

easy

A. HEXISTS user:1 age

B. HEXISTS age user:1

C. HEXISTS user:1:age

D. HEXISTS user:1, age

Solution

1. Step 1: Recall HEXISTS command syntax

   The correct syntax is HEXISTS key field, where key is the hash key and field is the field name.

2. Step 2: Match syntax with options

   HEXISTS user:1 age matches the correct syntax: HEXISTS user:1 age. Other options have wrong order or invalid format.

3. Final Answer:

   HEXISTS user:1 age -> Option A

4. Quick Check:

   HEXISTS key field syntax = C [OK]

Hint: Remember HEXISTS key field order exactly [OK]

Common Mistakes:

• Swapping key and field positions
• Using colon between key and field
• Adding commas between arguments

3. Given the Redis hash user:2 with fields {"name": "Alice", "email": "[e-mail]"}, what will HEXISTS user:2 phone return?

medium

A. 1

B. Error

C. 0

D. nil

Solution

1. Step 1: Check if field 'phone' exists in hash user:2

   The hash has fields 'name' and 'email' only. 'phone' is not present.

2. Step 2: Understand HEXISTS return value

   HEXISTS returns 1 if field exists, 0 if it does not. Since 'phone' is missing, it returns 0.

3. Final Answer:

   0 -> Option C

4. Quick Check:

   Field missing returns 0 = D [OK]

Hint: HEXISTS returns 0 for missing fields [OK]

Common Mistakes:

• Expecting nil instead of 0
• Thinking it returns an error for missing fields
• Confusing with HGET which returns nil

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

medium

A. The field name is misspelled

B. The key user:3 exists but is not a hash

C. HEXISTS requires two fields to check

D. Redis server is down

Solution

1. Step 1: Understand HEXISTS error causes

   HEXISTS returns error if the key exists but is not a hash type. If key does not exist, it returns 0, no error.

2. Step 2: Eliminate distractors

   Misspelled field returns 0, no error. HEXISTS checks one field. Server down causes connection failure, not command error. Wrong key type causes WRONGTYPE error.

3. Final Answer:

   The key user:3 exists but is not a hash -> Option B

4. Quick Check:

   Wrong key type causes HEXISTS error = A [OK]

Hint: Check key type before HEXISTS to avoid errors [OK]

Common Mistakes:

• Assuming misspelled field causes error
• Thinking HEXISTS needs multiple fields
• Ignoring key type mismatch

5. You want to add a field status to a Redis hash order:100 only if it does not already exist. Which sequence of commands correctly uses HEXISTS to check before adding?

hard

A. HEXISTS order:100 status
IF 0 THEN HSET order:100 status 'pending' END

B. HSET order:100 status 'pending'
HEXISTS order:100 status

C. HEXISTS order:100 status
IF 1 THEN HSET order:100 status 'pending' END

D. HGET order:100 status
IF nil THEN HSET order:100 status 'pending' END

Solution

1. Step 1: Understand the goal

   You want to add 'status' only if it does not exist, so first check with HEXISTS if field is missing (returns 0).

2. Step 2: Analyze command sequences

   HEXISTS order:100 status IF 0 THEN HSET order:100 status 'pending' END checks HEXISTS, then if result is 0 (field missing), it sets the field. This matches the requirement. HSET order:100 status 'pending' HEXISTS order:100 status sets field unconditionally first. HEXISTS order:100 status IF 1 THEN HSET order:100 status 'pending' END sets field if HEXISTS returns 1 (field exists), which is wrong. HGET order:100 status IF nil THEN HSET order:100 status 'pending' END uses HGET and nil check, which is valid but not using HEXISTS as asked.

3. Final Answer:

   HEXISTS order:100 status IF 0 THEN HSET order:100 status 'pending' END -> Option A

4. Quick Check:

   Check HEXISTS=0 before HSET = A [OK]

Hint: Add field only if HEXISTS returns 0 [OK]

Common Mistakes:

• Setting field without checking existence
• Checking HEXISTS=1 to add field
• Using HGET instead of HEXISTS when asked