HMSET and HMGET for bulk in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this HMGET command?

Given the following Redis commands:

HMSET user:1 name "Alice" age "30" city "Paris"
HMGET user:1 name city

What is the output of the HMGET command?

Redis

HMSET user:1 name "Alice" age "30" city "Paris"
HMGET user:1 name city

A ["Alice", null]

B ["Alice", "Paris"]

C ["name", "city"]

D ["30", "Paris"]

💡 Hint

HMGET returns the values of the specified fields in the order requested.

HMSET sets multiple fields in a hash. HMGET retrieves the values of the specified fields. Here, 'name' and 'city' fields are requested, so their values 'Alice' and 'Paris' are returned in a list.

📝 Syntax

intermediate

Which HMSET command is syntactically correct?

Choose the correct syntax for setting multiple fields in a Redis hash with HMSET for key 'user:2' with fields 'name' as 'Bob' and 'age' as '25'.

A HMSET user:2 name Bob age 25

B HMSET user:2 {name: Bob, age: 25}

C HMSET user:2 [name Bob age 25]

D HMSET user:2 (name Bob, age 25)

💡 Hint

HMSET expects key followed by field and value pairs separated by spaces.

The correct syntax for HMSET is: HMSET key field1 value1 field2 value2 ... Option A follows this format exactly.

❓ optimization

advanced

Which approach is best to retrieve multiple fields efficiently?

You want to get the values of fields 'name', 'age', 'city', and 'email' from a Redis hash 'user:3'. Which command is the most efficient to retrieve all these fields at once?

A HGETALL user:3

B

HGET user:3 name
HGET user:3 age
HGET user:3 city
HGET user:3 email

C GET user:3

D HMGET user:3 name age city email

💡 Hint

HMGET retrieves multiple specified fields in one command.

HMGET is designed to get multiple fields in one call, making it more efficient than multiple HGET calls. HGETALL returns all fields but may return unnecessary data. GET is for string keys, not hashes.

🔧 Debug

advanced

Why does this HMGET command return null for one field?

Given these commands:

HMSET user:4 name "Eve" age "28"
HMGET user:4 name city

Why does the HMGET command return ["Eve", null]?

Redis

HMSET user:4 name "Eve" age "28"
HMGET user:4 name city

A The field 'city' does not exist in the hash, so HMGET returns null for it.

B The HMGET command syntax is incorrect, causing null to be returned.

C Redis hashes cannot store more than two fields, so 'city' is ignored.

D The HMSET command failed to set any fields, so all values are null.

💡 Hint

HMGET returns null for fields that do not exist in the hash.

Since 'city' was never set in the hash 'user:4', HMGET returns null for that field while returning the value for 'name'.

🧠 Conceptual

expert

What is the main difference between HMSET and HSET in recent Redis versions?

Redis introduced HSET to replace HMSET. What is the key difference between HMSET and HSET when setting multiple fields in a hash?

A HMSET supports setting fields with expiration, HSET does not.

B HMSET can set multiple fields but HSET can only set one field at a time.

C HSET can set multiple fields at once and HMSET is deprecated but both behave similarly.

D HSET requires JSON format for fields, HMSET uses plain key-value pairs.

💡 Hint

Check Redis documentation about HMSET deprecation and HSET enhancements.

In recent Redis versions, HMSET is deprecated. HSET now supports setting multiple fields at once, making it the preferred command.

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