Why hashes represent objects in Redis - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

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

Redis

HSET user:1000 [1] Alice

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

Aname

Buser

C1000

Dfield

Attempts:

3 left

💡 Hint

Common Mistakes

Using the hash key instead of the field name.

Confusing the field with the value.

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

2fill in blank

medium

Complete the code to retrieve the value of the field 'age' from the Redis hash 'user:1000'.

Redis

HGET user:1000 [1]

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

Aage

Buser

C1000

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using the hash key instead of the field name.

Using a value instead of a field.

HGET fetches the value of a specific field in a Redis hash. Here, 'age' is the field name.

3fill in blank

hard

Fix the error in the code to get all fields and values from the hash 'user:1000'.

Redis

[1] user:1000

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

AHGET

BGETALL

CGET

DHGETALL

Attempts:

3 left

💡 Hint

Common Mistakes

Using GET or GETALL which are for strings, not hashes.

Using HGET which returns only one field.

HGETALL returns all fields and values in a Redis hash. Other commands do not work for hashes.

4fill in blank

hard

Fill both blanks to set multiple fields 'name' and 'age' in the hash 'user:1000'.

Redis

HMSET user:1000 [1] [2]

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

Aname Alice

Bage 30

Cname Bob

Dage 25

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong field names or values.

Putting both field and value in one option incorrectly.

HMSET sets multiple fields in a hash. Here, 'name Alice' and 'age 30' set two properties.

5fill in blank

hard

Fill all three blanks to create a hash 'user:1000' with fields 'name', 'age', and 'email'.

Redis

HMSET user:1000 [1] [2] [3]

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

Aname Alice

Bage 30

Cemail [e-mail]

Dphone 123456

Attempts:

3 left

💡 Hint

Common Mistakes

Including unrelated fields like 'phone'.

Mixing field names and values incorrectly.

HMSET sets multiple fields in a hash. Here, we set 'name', 'age', and 'email' fields to represent the object.

Practice

1. Why are Redis hashes considered similar to objects in programming?

easy

A. Because they store multiple related fields under one key, like object properties

B. Because they only store single values like simple variables

C. Because they automatically run code like functions

D. Because they store data in a list format

Solution

1. Step 1: Understand what a Redis hash stores

   A Redis hash stores multiple field-value pairs under one key, grouping related data.

2. Step 2: Compare with programming objects

   Programming objects group properties (fields) together, similar to how hashes group fields.

3. Final Answer:

   Because they store multiple related fields under one key, like object properties -> Option A

4. Quick Check:

   Hashes = grouped fields like object properties [OK]

Hint: Think of hashes as objects holding many properties [OK]

Common Mistakes:

• Confusing hashes with simple string keys
• Thinking hashes run code like functions
• Assuming hashes store lists instead of field-value pairs

2. Which of the following is the correct Redis command to set a field 'name' with value 'Alice' in a hash 'user:1'?

easy

A. HMSET user:1 name Alice

B. SET user:1 name Alice

C. HGET user:1 name Alice

D. HSET user:1 name Alice

Solution

1. Step 1: Identify the command to set a single field in a hash

   HSET sets a single field-value pair in a Redis hash.

2. Step 2: Check other commands

   SET is for strings, HMSET is deprecated, HGET retrieves values.

3. Final Answer:

   HSET user:1 name Alice -> Option D

4. Quick Check:

   Use HSET to set hash fields [OK]

Hint: Use HSET to add or update a single hash field [OK]

Common Mistakes:

• Using SET instead of HSET for hashes
• Using HGET which reads but does not set
• Using deprecated HMSET command

3. Given the Redis commands:
HSET user:2 age 30
HSET user:2 city "New York"
What will HGETALL user:2 return?

medium

A. ["user:2", "age", "30", "city", "New York"]

B. ["age", "30", "city", "New York"]

C. ["age", 30, "city", "New York"]

D. ["age", "city", "30", "New York"]

Solution

1. Step 1: Understand HGETALL output format

   HGETALL returns an array of field and value strings alternating.

2. Step 2: Match fields and values

   Fields are "age" and "city" with values "30" and "New York" as strings.

3. Final Answer:

   ["age", "30", "city", "New York"] -> Option B

4. Quick Check:

   HGETALL returns field-value pairs as strings [OK]

Hint: HGETALL returns fields and values as string pairs [OK]

Common Mistakes:

• Including the key name in the output
• Returning values as numbers instead of strings
• Mixing field and value order

4. You run the command HSET user:3 name "Bob" age 25 but get an error. What is the likely cause?

medium

A. Field names cannot be strings

B. The key 'user:3' does not exist yet

C. HSET only accepts one field-value pair at a time in this Redis version

D. Age must be a string, not a number

Solution

1. Step 1: Check HSET command syntax

   Older Redis versions allow only one field-value pair per HSET call.

2. Step 2: Analyze error cause

   Trying to set two pairs at once causes syntax error in older versions.

3. Final Answer:

   HSET only accepts one field-value pair at a time in this Redis version -> Option C

4. Quick Check:

   HSET single pair per call in older Redis [OK]

Hint: Check Redis version for multi-field HSET support [OK]

Common Mistakes:

• Assuming key must exist before HSET
• Thinking field names can't be strings
• Confusing data type requirements for values

5. You want to store user profiles with fields: 'name', 'age', and 'email' in Redis. Which approach best uses hashes to represent these profiles efficiently?

hard

A. Use a separate hash for each user with key like 'user:<id>' and fields for each property

B. Store all users in one big hash with user IDs as fields and JSON strings as values

C. Use simple string keys for each property like 'name:<id>', 'age:<id>', 'email:<id>'

D. Store user data as a list with all properties concatenated as one string

Solution

1. Step 1: Consider data organization for user profiles

   Each user should have a separate hash to group their properties clearly.

2. Step 2: Evaluate other options

   One big hash mixes users, separate strings scatter data, lists lose field names.

3. Final Answer:

   Use a separate hash for each user with key like 'user:<id>' and fields for each property -> Option A

4. Quick Check:

   Separate hashes per user = clear, efficient object representation [OK]

Hint: Use one hash per object for clear property grouping [OK]

Common Mistakes:

• Mixing all users in one hash losing clarity
• Using separate keys for each property causing scattered data
• Storing data as lists losing field names