Object storage with hashes in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set a field 'name' with value 'Alice' in a Redis hash called 'user:1'.

Redis

HSET user:1 [1] Alice

Drag options to blanks, or click blank then click option'

Aname

Buser

Cvalue

Dfield

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'user' instead of the field name.

Confusing the field with the hash key.

The HSET command sets a field in a hash. Here, the field is 'name'.

2fill in blank

medium

Complete the code to get the value of the 'email' field from the Redis hash 'user:2'.

Redis

HGET user:2 [1]

Drag options to blanks, or click blank then click option'

Apassword

Bid

Cemail

Dusername

Attempts:

3 left

💡 Hint

Common Mistakes

Using the hash key instead of the field name.

Using a wrong field like 'username' or 'id'.

The HGET command retrieves the value of a specific field in a hash. Here, we want the 'email' field.

3fill in blank

hard

Fix the error in the code to delete the 'age' field from the Redis hash 'user:3'.

Redis

HDEL user:3 [1]

Drag options to blanks, or click blank then click option'

Auser:3

Bfield

Cdelete

Dage

Attempts:

3 left

💡 Hint

Common Mistakes

Using the hash key instead of the field name.

Using a command word like 'delete' as a field.

The HDEL command deletes one or more fields from a hash. The field to delete here is 'age'.

4fill in blank

hard

Fill both blanks to set the 'city' field to 'Paris' in the hash 'user:4' and then retrieve it.

Redis

HSET user:4 [1] Paris
HGET user:4 [2]

Drag options to blanks, or click blank then click option'

Acity

Bname

Dlocation

Attempts:

3 left

💡 Hint

Common Mistakes

Using different field names in set and get commands.

Using the hash key instead of the field name.

Both commands use the field 'city' to set and get the value in the hash.

5fill in blank

hard

Fill all three blanks to create a hash 'user:5' with fields 'name' and 'age', then retrieve the 'age' field.

Redis

HSET user:5 [1] John [2] 30
HGET user:5 [3]

Drag options to blanks, or click blank then click option'

Aname

Bage

Dusername

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up field names or using wrong fields.

Trying to get a field that was not set.

The HSET command sets multiple fields: 'name' with 'John' and 'age' with '30'. Then HGET retrieves the 'age' field.

Practice

1. What does a Redis hash store?

easy

A. A set of unique keys without duplicates

B. Only a single string value per key

C. A list of keys without values

D. Multiple fields and their values under one key

Solution

1. Step 1: Understand Redis hash structure

   Redis hashes store multiple field-value pairs grouped under a single key.

2. Step 2: Compare options with hash definition

   Only Multiple fields and their values under one key correctly describes storing multiple fields and values under one key.

3. Final Answer:

   Multiple fields and their values under one key -> Option D

4. Quick Check:

   Hashes = multiple fields + values [OK]

Hint: Hashes group fields and values under one key [OK]

Common Mistakes:

• Thinking hashes store only one value
• Confusing hashes with lists or sets
• Assuming keys are stored without values

2. Which Redis command correctly adds a field 'name' with value 'Alice' to a hash key 'user:1'?

easy

A. HGET user:1 name Alice

B. HSET user:1 name Alice

C. SET user:1 name Alice

D. HMGET user:1 name Alice

Solution

1. Step 1: Identify command to add/update hash field

   HSET is the Redis command to set a field in a hash.

2. Step 2: Check command syntax

   HSET key field value is correct; so HSET user:1 name Alice is valid.

3. Final Answer:

   HSET user:1 name Alice -> Option B

4. Quick Check:

   HSET adds/updates hash fields [OK]

Hint: Use HSET to add or update hash fields [OK]

Common Mistakes:

• Using HGET instead of HSET to add data
• Using SET which is for strings, not hashes
• Confusing HMGET (retrieves multiple fields) with HSET

3. Given the following Redis commands executed in order:
HSET user:2 age 30
HSET user:2 name Bob
HGETALL user:2
What is the output of HGETALL user:2?

medium

A. ["age", 30, "name", "Bob"]

B. ["user:2", "age", "30", "name", "Bob"]

C. ["age", "30", "name", "Bob"]

D. ["name", "Bob"]

Solution

1. Step 1: Understand HSET commands

   HSET adds fields 'age' with '30' and 'name' with 'Bob' to hash 'user:2'.

2. Step 2: Understand HGETALL output format

   HGETALL returns an array of field-value pairs as strings in order added.

3. Final Answer:

   ["age", "30", "name", "Bob"] -> Option C

4. Quick Check:

   HGETALL returns all fields and values as string pairs [OK]

Hint: HGETALL returns flat list of field-value strings [OK]

Common Mistakes:

• Expecting key name in output
• Mixing data types (number vs string)
• Only getting one field instead of all

4. You run the command HGET user:3 email but get a null response. What could be the reason?

medium

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

B. The key 'user:3' is a string, not a hash

C. The Redis server is down

D. The command syntax is incorrect

Solution

1. Step 1: Understand HGET behavior

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

2. Step 2: Check other options

   Key being a string would cause an error, not null; server down or syntax error would cause different errors.

3. Final Answer:

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

4. Quick Check:

   Missing field in hash returns null [OK]

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

Common Mistakes:

• Assuming null means server or syntax error
• Confusing key type errors with missing field
• Expecting error instead of null for missing field

5. You want to store user profiles with fields 'name', 'age', and 'email' in Redis hashes. Which approach is best to retrieve all user data efficiently?

hard

A. Store each user as a separate hash with key 'user:<id>' and use HGETALL to get all fields

B. Store all users in one hash with fields like 'user1_name', 'user1_age', 'user2_name', etc.

C. Store each field as a separate key like 'name:<id>', 'age:<id>', 'email:<id>'

D. Store user data as JSON strings in simple keys

Solution

1. Step 1: Analyze storage options

   Storing each user as a separate hash groups related fields logically and allows efficient access.

2. Step 2: Evaluate retrieval efficiency

   Using HGETALL on 'user:<id>' fetches all fields at once, simpler and faster than scattered keys or complex parsing.

3. Final Answer:

   Store each user as a separate hash with key 'user:<id>' and use HGETALL to get all fields -> Option A

4. Quick Check:

   Separate hashes per user = organized and efficient [OK]

Hint: Use one hash per user for grouped fields and easy retrieval [OK]

Common Mistakes:

• Mixing all users in one hash causing complex keys
• Using separate keys for each field complicates retrieval
• Storing JSON loses Redis hash benefits