HEXISTS for field check in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the field 'name' exists in the hash 'user:1000'.

Redis

HEXISTS user:1000 [1]

Drag options to blanks, or click blank then click option'

Aaddress

Bage

Cname

Demail

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong field name will return 0 even if the hash exists.

Confusing the hash key with the field name.

The HEXISTS command checks if a specific field exists in a hash. Here, we want to check the field name.

2fill in blank

medium

Complete the code to check if the field 'email' exists in the hash 'user:2000'.

Redis

HEXISTS [1] email

Drag options to blanks, or click blank then click option'

Auser:1000

Buser:2000

Cuser:3000

Duser:4000

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong user key will cause the check to fail.

Swapping the field and key arguments.

The command checks the field email in the hash user:2000.

3fill in blank

hard

Fix the error in the command to check if the field 'phone' exists in the hash 'contact:500'.

Redis

HEXISTS contact:500 [1]

Drag options to blanks, or click blank then click option'

Aphone

Bphones

Cphone_number

Dcontact

Attempts:

3 left

💡 Hint

Common Mistakes

Using a similar but incorrect field name.

Adding extra words or underscores.

The field name must exactly match the stored field. Here, it is phone.

4fill in blank

hard

Fill both blanks to check if the field 'status' exists in the hash 'order:1234'.

Redis

HEXISTS [1] [2]

Drag options to blanks, or click blank then click option'

Aorder:1234

Buser:1234

Cstatus

Dstate

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the hash key and field.

Using incorrect field or key names.

The command checks the field status in the hash order:1234.

5fill in blank

hard

Fill all three blanks to check if the field 'price' exists in the hash 'product:789' using the Redis CLI command.

Redis

[1] [2] [3]

Drag options to blanks, or click blank then click option'

AHEXISTS

Bproduct:789

Cprice

DHGET

Attempts:

3 left

💡 Hint

Common Mistakes

Using HGET instead of HEXISTS.

Swapping the order of arguments.

The Redis CLI command to check a field in a hash is HEXISTS followed by the hash key and the field name.

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