HSET and HGET for fields in Redis

Introduction

HSET and HGET let you store and get small pieces of information inside a bigger group, like putting notes in labeled folders.

You want to save user details like name and age under one user ID.

You need to update or read a specific setting in a configuration without changing others.

You want to keep track of scores for different players in a game.

You want to store product details like price and stock count under one product ID.

Syntax

Redis

HSET key field value
HGET key field

HSET adds or updates a field with a value inside a key that holds many fields.

HGET retrieves the value of a specific field inside the key.

Examples

Stores the name 'Alice' in the 'name' field under the key 'user:1'.

Redis

HSET user:1 name "Alice"

Gets the value stored in the 'name' field under 'user:1', which is 'Alice'.

Redis

HGET user:1 name

Sets the price field to 29.99 for the product with key 'product:100'.

Redis

HSET product:100 price 29.99

Retrieves the price of the product 'product:100'.

Redis

HGET product:100 price

Sample Program

This example stores age and city for user 42, then retrieves both values.

Redis

HSET user:42 age 30
HSET user:42 city "New York"
HGET user:42 age
HGET user:42 city

Important Notes

If the field does not exist, HGET returns nil (no value).

HSET creates the key if it does not exist already.

You can update a field by calling HSET again with the same field name.

Summary

HSET stores or updates a field inside a key.

HGET retrieves the value of a specific field.

They help organize related data under one key like a mini-database.

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