HSET and HGET for fields in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this Redis command sequence?

Consider the following Redis commands executed in order:

HSET user:1 name "Alice" age 30
HGET user:1 name

What will be the output of the HGET command?

Redis

HSET user:1 name "Alice" age 30
HGET user:1 name

A Alice

B nil

C 30

D Error: wrong type

💡 Hint

HSET stores fields and values in a hash. HGET retrieves the value of a given field.

The HSET command sets multiple fields in the hash 'user:1'. The field 'name' is set to 'Alice'. The HGET command retrieves the value of the 'name' field, which is 'Alice'.

❓ query_result

intermediate

What happens if you HGET a non-existing field?

Given the hash key user:2 with fields:
HSET user:2 name "Bob"

What is the result of:
HGET user:2 age?

Redis

HSET user:2 name "Bob"
HGET user:2 age

A nil

B "" (empty string)

C 0

D Error: field not found

💡 Hint

HGET returns nil if the field does not exist in the hash.

Since the field 'age' does not exist in the hash 'user:2', HGET returns nil to indicate no value.

📝 Syntax

advanced

Which command correctly sets multiple fields in a hash?

You want to set the fields 'name' to 'Carol' and 'age' to 25 in the hash 'user:3'. Which command is correct?

A HSET user:3 (name, Carol, age, 25)

B HSET user:3 name Carol age 25

C HSET user:3 [name, Carol, age, 25]

D HSET user:3 {name: Carol, age: 25}

💡 Hint

HSET takes the key followed by field and value pairs as separate arguments.

The correct syntax for HSET is: HSET key field1 value1 field2 value2 ... Option B follows this syntax exactly.

❓ optimization

advanced

Which approach is more efficient to update multiple fields in a Redis hash?

You need to update 5 fields in a Redis hash. Which is the best approach?

A Use HGET to get all fields, modify locally, then HSET all fields again

B Use 5 separate HSET commands, one per field

C Use one HSET command with all 5 field-value pairs at once

D Use multiple HGET commands, one per field, then HSET for each

💡 Hint

Minimizing the number of commands sent to Redis improves performance.

Using a single HSET command with all fields reduces network overhead and is more efficient than multiple commands.

🧠 Conceptual

expert

What is the result of this Redis command sequence?

Given the commands:
HSET user:4 name "Dave" age 40
HSET user:4 age 41
HGET user:4 age

What is the output of the last command?

Redis

HSET user:4 name "Dave" age 40
HSET user:4 age 41
HGET user:4 age

A "40"

B Error: field not found

C nil

D "41"

💡 Hint

HSET overwrites existing fields with new values.

The second HSET updates the 'age' field to 41, so HGET returns "41".

Practice

1. What does the Redis command HSET do?

easy

A. Stores or updates a field and its value inside a hash key

B. Retrieves the value of a field from a hash key

C. Deletes a field from a hash key

D. Lists all keys in the database

Solution

1. Step 1: Understand the purpose of HSET

   The HSET command is used to store or update a field-value pair inside a Redis hash key, organizing related data together.

2. Step 2: Differentiate from other commands

   HGET retrieves values, HDEL deletes fields, and KEYS lists keys, so they do not match the description of HSET.

3. Final Answer:

   Stores or updates a field and its value inside a hash key -> Option A

4. Quick Check:

   HSET stores/updates field = A [OK]

Hint: HSET adds or changes a field inside a hash key [OK]

Common Mistakes:

• Confusing HSET with HGET
• Thinking HSET deletes fields
• Mixing HSET with key listing commands

2. Which of the following is the correct syntax to set field name to John in hash key user:1 using Redis?

easy

A. HSET user:1 name John

B. HGET user:1 name John

C. SET user:1 name John

D. HSET name user:1 John

Solution

1. Step 1: Recall HSET syntax

   The correct syntax is HSET key field value. Here, key is user:1, field is name, and value is John.

2. Step 2: Check each option

   HSET user:1 name John matches the syntax exactly. HGET user:1 name John uses HGET which is for retrieval. SET user:1 name John uses SET which is for string keys, not hashes. HSET name user:1 John swaps field and key, which is incorrect.

3. Final Answer:

   HSET user:1 name John -> Option A

4. Quick Check:

   HSET key field value = C [OK]

Hint: Remember: HSET key field value order [OK]

Common Mistakes:

• Using HGET instead of HSET to set values
• Swapping field and key positions
• Using SET for hash fields

3. Given these Redis commands executed in order:
HSET user:2 age 30
HSET user:2 city London
HGET user:2 age
What is the output of the last command?

medium

A. London

B. 30

C. user:2

D. nil

Solution

1. Step 1: Analyze the HSET commands

   The first command sets field age to 30 in user:2. The second sets city to London.

2. Step 2: Understand HGET command

   HGET user:2 age retrieves the value of field age from user:2, which was set to 30.

3. Final Answer:

   30 -> Option B

4. Quick Check:

   HGET user:2 age = 30 [OK]

Hint: HGET returns the value of the requested field [OK]

Common Mistakes:

• Confusing field names and values
• Expecting HGET to return the key name
• Assuming HGET returns all fields

4. You run the command HGET user:3 email but get (nil) as output. What could be the reason?

medium

A. The key user:3 is not a hash but a string

B. The Redis server is down

C. The field email does not exist in the hash user:3

D. The syntax of HGET is incorrect

Solution

1. Step 1: Understand (nil) output in HGET

   When HGET returns (nil), it means the requested field does not exist in the hash key.

2. Step 2: Evaluate other options

   If the key was not a hash, Redis would return a type error, not (nil). Server down would cause connection errors, and incorrect syntax would cause errors, not (nil).

3. Final Answer:

   The field email does not exist in the hash user:3 -> Option C

4. Quick Check:

   HGET nil means field missing = B [OK]

Hint: Nil means field missing, not syntax or server error [OK]

Common Mistakes:

• Assuming (nil) means server or syntax error
• Confusing missing field with missing key
• Ignoring data type errors

5. You want to store user profile data in Redis using a hash key profile:100. You run:
HSET profile:100 name Alice
HSET profile:100 age 0
HGET profile:100 age
What will the last command return and why?

hard

A. Empty string, because age is unset

B. nil, because zero is treated as a missing value

C. Error, because age cannot be zero

D. 0, because Redis stores numeric zero as a valid string value

Solution

1. Step 1: Understand storing zero in Redis hash

   Redis stores all hash field values as strings. The value 0 is stored as the string "0", which is a valid value.

2. Step 2: Check HGET behavior for zero value

   HGET returns the exact stored string. Since age was set to 0, it returns "0", not nil or empty.

3. Final Answer:

   0, because Redis stores numeric zero as a valid string value -> Option D

4. Quick Check:

   HGET returns stored string '0' = D [OK]

Hint: Zero is stored as string '0', not nil or empty [OK]

Common Mistakes:

• Thinking zero is treated as missing or false
• Expecting errors for numeric zero
• Confusing empty string with zero