HMSET and HMGET for bulk in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Redis command HMSET do?

HMSET sets multiple fields and their values in a hash stored at a key in Redis. It allows you to add or update many fields at once.

beginner

How do you retrieve multiple fields from a Redis hash at once?

You use the HMGET command, which gets the values of specified fields from a hash key in one call.

beginner

Example: What does HMSET user:1 name "Alice" age "30" city "NY" do?

It creates or updates the hash at key user:1 with fields name, age, and city set to "Alice", "30", and "NY" respectively.

beginner

What will HMGET user:1 name city return if the hash contains those fields?

It returns an array of values for the fields name and city, for example: ["Alice", "NY"].

intermediate

Why use HMSET and HMGET instead of multiple HSET or HGET calls?

Because HMSET and HMGET reduce the number of commands sent to Redis, making operations faster and more efficient when working with many fields.

What does the Redis command HMSET do?

A Gets multiple fields from a hash

B Creates a new database

C Deletes a hash key

D Sets multiple fields in a hash at once

HMSET sets multiple fields and values in a Redis hash key.

Which command retrieves multiple fields from a Redis hash in one call?

A HMGET

B HDEL

C HGETALL

D HSET

HMGET fetches specified fields from a hash key.

If you want to set fields name and age in a hash key user:2, which command is correct?

A HGET user:2 name

B HMGET user:2 name age

C HMSET user:2 name "Bob" age "25"

D DEL user:2

HMSET sets multiple fields in a hash key.

What does HMGET user:1 name city return if user:1 has those fields?

A A single string with both values

B An array of values for <code>name</code> and <code>city</code>

C Deletes the fields

D An error

HMGET returns an array of values for requested fields.

Why is using HMSET and HMGET better than multiple HSET or HGET calls?

A They reduce network calls and improve performance

B They create new databases

C They delete the hash

D They are slower

Using bulk commands reduces the number of calls to Redis, improving speed.

Explain how HMSET and HMGET work together to manage multiple fields in a Redis hash.

Think about setting and getting many fields at once.

Describe a real-life example where using HMSET and HMGET would be helpful.

Imagine a user profile with name, age, city, and more.

Practice

1. What does the Redis command HMSET do?

easy

A. Retrieves all keys from the database.

B. Deletes a hash key and all its fields.

C. Stores multiple fields and their values in a single hash key at once.

D. Increments a numeric value in a hash field.

Solution

1. Step 1: Understand HMSET purpose

   The HMSET command is used to save many fields and their values inside one hash key in Redis.

2. Step 2: Compare with other commands

   Other commands like DEL delete keys, and INCR increments values, so they do not match HMSET's bulk saving role.

3. Final Answer:

   Stores multiple fields and their values in a single hash key at once. -> Option C

4. Quick Check:

   HMSET saves many fields at once = B [OK]

Hint: HMSET saves many fields in one hash key at once [OK]

Common Mistakes:

• Confusing HMSET with commands that delete keys
• Thinking HMSET retrieves data instead of saving
• Mixing HMSET with single field commands like HSET

2. Which of the following is the correct syntax to set multiple fields in a hash named user:1 using HMSET?

easy

A. HMSET user:1 ["name", "Alice", "age", 30, "city", "NY"]

B. HMSET user:1 (name, "Alice", age, 30, city, "NY")

C. HMSET user:1 {name: "Alice", age: 30, city: "NY"}

D. HMSET user:1 name "Alice" age 30 city "NY"

Solution

1. Step 1: Recall HMSET syntax

   HMSET expects the key followed by field and value pairs separated by spaces, without brackets or braces.

2. Step 2: Check each option

   HMSET user:1 name "Alice" age 30 city "NY" matches the correct syntax: key, field, value, field, value... Options B, C, and D use invalid brackets or JSON-like syntax not accepted by Redis.

3. Final Answer:

   HMSET user:1 name "Alice" age 30 city "NY" -> Option D

4. Quick Check:

   HMSET syntax is key + field value pairs space-separated = A [OK]

Hint: HMSET uses space-separated field-value pairs, no brackets [OK]

Common Mistakes:

• Using brackets or braces in HMSET command
• Putting fields and values inside parentheses
• Confusing HMSET with JSON syntax

3. Given the following commands executed in Redis:
HMSET product:100 name "Laptop" price 1200 stock 30
HMGET product:100 name price
What will be the output of the HMGET command?

medium

A. ["Laptop", "1200"]

B. ["Laptop", 1200]

C. ["name", "price"]

D. ["Laptop"]

Solution

1. Step 1: Understand HMSET stored data

   The HMSET command stores fields 'name' with value "Laptop" and 'price' with value "1200" (as string) in the hash 'product:100'.

2. Step 2: Understand HMGET output

   HMGET fetches the values of the requested fields in order, returning an array of strings: ["Laptop", "1200"].

3. Final Answer:

   ["Laptop", "1200"] -> Option A

4. Quick Check:

   HMGET returns values as strings in array = A [OK]

Hint: HMGET returns values as strings in array order [OK]

Common Mistakes:

• Expecting numeric types instead of strings
• Confusing HMGET output with field names
• Getting only one value instead of array

4. You run the command HMGET user:5 name age but get (nil) for both fields. What is the most likely reason?

medium

A. Redis server is down.

B. The hash key user:5 does not exist or has no such fields.

C. HMGET cannot fetch multiple fields at once.

D. HMGET syntax requires brackets around field names.

Solution

1. Step 1: Interpret (nil) results

   Getting (nil) means the requested fields or the hash key do not exist in Redis.

2. Step 2: Check command syntax and server status

   HMGET syntax is correct without brackets, and if the server was down, no response would be received.

3. Final Answer:

   The hash key user:5 does not exist or has no such fields. -> Option B

4. Quick Check:

   (nil) means missing key or fields = D [OK]

Hint: (nil) means key or fields missing, not syntax error [OK]

Common Mistakes:

• Adding brackets around fields in HMGET
• Assuming HMGET cannot fetch multiple fields
• Thinking server down causes (nil) responses

5. You want to store user profile data with fields name, email, and age in Redis using HMSET, then retrieve only email and age with HMGET. Which sequence of commands is correct?

hard

A. HMSET user:10 name "Bob" email "[e-mail]" age 25HMGET user:10 email age

B. HMSET user:10 {name: "Bob", email: "[e-mail]", age: 25}HMGET user:10 [email, age]

C. HMSET user:10 (name "Bob", email "[e-mail]", age 25)HMGET user:10 (email age)

D. HMSET user:10 name="Bob" email="[e-mail]" age=25HMGET user:10 email age

Solution

1. Step 1: Write correct HMSET command

   HMSET requires key followed by field-value pairs separated by spaces without brackets or equals signs, so HMSET user:10 name "Bob" email "[e-mail]" age 25
   HMGET user:10 email age is correct.

2. Step 2: Write correct HMGET command

   HMGET takes the key and list of fields separated by spaces, so HMSET user:10 name "Bob" email "[e-mail]" age 25
   HMGET user:10 email age's HMGET is correct.

3. Final Answer:

   HMSET user:10 name "Bob" email "[e-mail]" age 25
   HMGET user:10 email age -> Option A

4. Quick Check:

   HMSET and HMGET use space-separated fields and values = C [OK]

Hint: Use space-separated fields and values, no brackets or equals [OK]

Common Mistakes:

• Using JSON or bracket syntax in HMSET
• Adding parentheses around fields in HMGET
• Using equals signs in HMSET command