HKEYS and HVALS in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What does HKEYS return?

Given a Redis hash stored as HSET user:1 name "Alice" age "30" city "Paris", what is the output of HKEYS user:1?

Redis

HSET user:1 name "Alice" age "30" city "Paris"
HKEYS user:1

A ["name", "age", "city"]

B ["Alice", "30", "Paris"]

C ["user:1"]

D []

💡 Hint

HKEYS returns all the field names (keys) inside the hash.

The HKEYS command returns a list of all the field names (keys) stored inside the hash. It does not return the values or the hash key itself.

❓ query_result

intermediate

What does HVALS return?

Given a Redis hash stored as HSET product:100 price "19.99" stock "50" category "books", what is the output of HVALS product:100?

Redis

HSET product:100 price "19.99" stock "50" category "books"
HVALS product:100

A ["19.99", "50", "books"]

B ["price", "stock", "category"]

C ["product:100"]

D []

💡 Hint

HVALS returns all the values inside the hash fields.

The HVALS command returns a list of all the values stored inside the hash fields. It does not return the field names or the hash key.

🧠 Conceptual

advanced

Difference between HKEYS and HVALS

Which statement correctly describes the difference between HKEYS and HVALS in Redis?

A <code>HKEYS</code> returns all keys in Redis; <code>HVALS</code> returns all values in Redis.

B <code>HKEYS</code> returns all field values; <code>HVALS</code> returns all field names.

C <code>HKEYS</code> returns all field names; <code>HVALS</code> returns all field values.

D <code>HKEYS</code> returns the hash key; <code>HVALS</code> returns the hash size.

💡 Hint

Think about what each command returns from inside the hash.

HKEYS returns the field names (keys) inside a hash, while HVALS returns the values of those fields. They do not return the hash key or all keys in Redis.

📝 Syntax

advanced

Identify the syntax error

Which of the following Redis commands will cause a syntax error?

A HVALS product:100

B HKEYS

C HKEYS user:1

D HVALS orders:2023

💡 Hint

Check if the command has the required arguments.

Both HKEYS and HVALS require a key argument. Option B misses the key, causing a syntax error.

❓ optimization

expert

Efficiently retrieving all fields and values

You want to retrieve all fields and their values from a large Redis hash named session:123. Which command is the most efficient to get both keys and values in one call?

A HKEYS session:123

B HVALS session:123

C HMGET session:123 *

D HGETALL session:123

💡 Hint

Think about a command that returns both keys and values together.

HGETALL returns all fields and their values in one call. HKEYS and HVALS return only keys or values separately. HMGET requires specific fields, not a wildcard.

Practice

1. What does the Redis command HKEYS myhash return?

easy

A. All the field names (keys) inside the hash myhash

B. All the values inside the hash myhash

C. The number of fields in the hash myhash

D. Deletes the hash myhash

Solution

1. Step 1: Understand the purpose of HKEYS

   The command HKEYS is used to get all the field names (keys) from a Redis hash.

2. Step 2: Apply to the given hash

   When you run HKEYS myhash, it returns all the keys inside myhash, not the values or count.

3. Final Answer:

   All the field names (keys) inside the hash myhash -> Option A

4. Quick Check:

   HKEYS returns keys = A [OK]

Hint: HKEYS always lists keys, not values or counts [OK]

Common Mistakes:

• Confusing HKEYS with HVALS
• Thinking it returns values instead of keys
• Assuming it returns the count of fields

2. Which of the following is the correct syntax to get all values from a Redis hash named user:100?

easy

A. HVALS user:100

B. HKEYS user:100

C. HGETALL user:100

D. HVALS(user:100)

Solution

1. Step 1: Identify the command to get all values

   The Redis command to get all values from a hash is HVALS.

2. Step 2: Check syntax correctness

   The correct syntax is HVALS keyname, so HVALS user:100 is correct. Parentheses are not used in Redis commands.

3. Final Answer:

   HVALS user:100 -> Option A

4. Quick Check:

   HVALS syntax = HVALS key [OK]

Hint: HVALS command syntax: HVALS key (no parentheses) [OK]

Common Mistakes:

• Using parentheses like a function call
• Mixing HKEYS and HVALS commands
• Using HGETALL instead of HVALS for values only

3. Given a Redis hash product:1 with fields and values:
{"name": "Pen", "color": "blue", "price": "1.5"}
What will HVALS product:1 return?

medium

A. ["name", "color", "price"]

B. ["Pen", "blue", "1.5"]

C. ["product:1"]

D. An error because HVALS needs a list

Solution

1. Step 1: Understand what HVALS returns

   HVALS returns all the values stored in the hash fields, not the keys.

2. Step 2: Apply to the given hash

   The values are "Pen", "blue", and "1.5". So the command returns these values as a list.

3. Final Answer:

   ["Pen", "blue", "1.5"] -> Option B

4. Quick Check:

   HVALS returns values list = A [OK]

Hint: HVALS returns only values, not keys or errors [OK]

Common Mistakes:

• Confusing keys with values
• Expecting field names instead of values
• Thinking HVALS returns the hash key name

4. You run HKEYS myhash but get an empty list even though you know myhash has data. What could be the problem?

medium

A. The Redis server is down

B. You typed HVALS myhash instead of HKEYS

C. myhash is not a hash type but a string or other type

D. The hash myhash is empty

Solution

1. Step 1: Check the data type of the key

   If myhash is not a hash but a string or other type, HKEYS returns an empty list.

2. Step 2: Understand why other options are incorrect

   Typing HVALS would return values, not empty list. Server down causes connection errors, not empty list. An empty hash would return empty list but question says it has data.

3. Final Answer:

   myhash is not a hash type but a string or other type -> Option C

4. Quick Check:

   Wrong type key = empty HKEYS result [OK]

Hint: Empty HKEYS often means key is wrong type, not empty hash [OK]

Common Mistakes:

• Assuming empty list means empty hash
• Ignoring key data type
• Thinking server down returns empty list

5. You want to list all field names and values from a Redis hash session:123 but only want the values that are not empty strings. Which approach using HKEYS and HVALS is best?

hard

A. Use HVALS session:123 and remove empty strings from the returned list

B. Use HKEYS session:123 only, empty values are not returned

C. Use HGETALL session:123 and filter out empty values in your application

D. Use HKEYS session:123 to get keys, then filter values by fetching each with HGET and checking for empty strings

Solution

1. Step 1: Understand what HKEYS and HVALS return

   HKEYS returns all field names, HVALS returns all values including empty strings.

2. Step 2: Choose a method to filter empty values

   Since HVALS returns all values without keys, filtering empty strings is hard without knowing which value belongs to which key. Using HKEYS to get keys, then fetching each value with HGET allows filtering empty strings precisely.

3. Step 3: Evaluate other options

   HVALS alone does not provide keys, so filtering loses context. HGETALL returns all keys and values but question asks about HKEYS and HVALS usage. Using only HKEYS does not get values.

4. Final Answer:

   Use HKEYS to get keys, then fetch and filter values with HGET -> Option D

5. Quick Check:

   Filter empty values by keys + HGET = B [OK]

Hint: Get keys first, then fetch values to filter empty strings [OK]

Common Mistakes:

• Filtering values without keys loses context
• Assuming HVALS excludes empty strings
• Using only HKEYS without fetching values