HMSET and HMGET for bulk in Redis - Step-by-Step Execution

Concept Flow - HMSET and HMGET for bulk

Start

↓

HMSET command

↓

Store multiple fields and values in hash

↓

HMGET command

↓

Retrieve multiple fields from hash

↓

Output values

↓

End

The flow shows storing multiple fields in a hash with HMSET, then retrieving them with HMGET, and finally outputting the values.

Execution Sample

Redis

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

Stores multiple fields for user:1 and then retrieves those fields in bulk.

Execution Table

Step	Command	Action	Hash Key	Fields/Values	Result/Output
1	HMSET user:1 name "Alice" age "30" city "NY"	Store multiple fields	user:1	name=Alice, age=30, city=NY	OK
2	HMGET user:1 name age city	Retrieve multiple fields	user:1	name, age, city	["Alice", "30", "NY"]
3	End	No more commands			Execution complete

💡 All requested fields retrieved; no more commands to execute.

Variable Tracker

Variable	Start	After HMSET	After HMGET	Final
user:1 hash	empty	{"name":"Alice", "age":"30", "city":"NY"}	unchanged	{"name":"Alice", "age":"30", "city":"NY"}
retrieved_values	none	none	["Alice", "30", "NY"]	["Alice", "30", "NY"]

Key Moments - 2 Insights

Why does HMGET return values in the same order as the fields requested?

HMGET returns values in the order of the fields you ask for, as shown in execution_table step 2, so you can match each value to its field easily.

What happens if a field requested by HMGET does not exist in the hash?

If a field is missing, HMGET returns nil (null) for that field in the output array, keeping the order intact. This is important to know when reading results.

Visual Quiz - 3 Questions

Look at the execution_table step 1, what does HMSET return after storing fields?

A An error message

B "OK"

C "nil"

D The stored values

💡 Hint

Check the Result/Output column in execution_table step 1.

At which step do we see the retrieved values from the hash?

A Step 2

B Step 1

C Step 3

D No step shows retrieval

💡 Hint

Look at the Action and Result/Output columns in execution_table.

If we add a new field 'email' with HMSET, how will variable 'user:1 hash' change after HMSET?

A It will stay empty

B It will remove existing fields

C It will include 'email' with its value

D It will only keep 'email'

💡 Hint

Refer to variable_tracker showing how fields are added without removing existing ones.

Concept Snapshot

HMSET key field1 value1 field2 value2 ...
- Stores multiple fields in a hash at once.
HMGET key field1 field2 ...
- Retrieves multiple fields from a hash in the order requested.
- Missing fields return nil.
Use these commands to efficiently handle many hash fields in bulk.

Full Transcript

This visual execution shows how HMSET stores multiple fields and values in a Redis hash key, and how HMGET retrieves those fields in bulk. Step 1 executes HMSET storing name, age, and city fields for user:1, returning OK. Step 2 runs HMGET to get those fields, returning their values in the same order. Variables track the hash contents and retrieved values. Key moments clarify why HMGET preserves order and what happens if fields are missing. The quiz tests understanding of command results and variable changes. The snapshot summarizes syntax and behavior for quick reference.

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