HDEL for field removal in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to remove a field named 'age' from the hash 'user:1000'.

Redis

HDEL user:1000 [1]

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

Aage

Bname

Cemail

Daddress

Attempts:

3 left

💡 Hint

Common Mistakes

Using the hash key instead of the field name.

Trying to remove a field that does not exist.

The HDEL command removes the specified field from the hash. Here, 'age' is the field to remove.

2fill in blank

medium

Complete the code to remove the field 'email' from the hash 'contact:1234'.

Redis

HDEL [1] email

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

Acontact:1234

Buser:1000

Cprofile:5678

Dsession:4321

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the hash key and field name positions.

Using a wrong or non-existing hash key.

The HDEL command requires the hash key first, then the field name. Here, 'contact:1234' is the hash key.

3fill in blank

hard

Fix the error in the command to remove the field 'phone' from the hash 'client:789'.

Redis

HDEL client:789 [1]

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

Aphones

Bphonenumber

CPhone

Dphone

Attempts:

3 left

💡 Hint

Common Mistakes

Using a plural or different variation of the field name.

Using incorrect capitalization.

The field name must exactly match the one stored in the hash. Here, the correct field is 'phone' in lowercase.

4fill in blank

hard

Fill both blanks to remove the field 'status' from the hash 'order:555'.

Redis

HDEL [1] [2]

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

Aorder:555

Buser:555

Cstatus

Dstate

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up hash key and field name.

Using incorrect field names.

The first blank is the hash key 'order:555', and the second blank is the field name 'status' to remove.

5fill in blank

hard

Fill both blanks to remove the field 'email' from the hash 'profile:2023' and confirm the command.

Redis

HDEL [1] [2] 

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

Aprofile:2023

Bemail

Attempts:

3 left

💡 Hint

Common Mistakes

Adding extra fields when not needed.

Leaving blanks empty when required.

The command syntax is HDEL key field [field ..]. Here, only one field 'email' is removed, so the third blank is empty (no extra field).

Practice

1. What does the Redis command HDEL do?

easy

A. Renames a hash key

B. Deletes specified fields from a hash

C. Retrieves all fields from a hash

D. Adds new fields to a hash

Solution

1. Step 1: Understand the purpose of HDEL

   The HDEL command is used to remove one or more fields from a Redis hash.

2. Step 2: Compare with other commands

   Other commands like HSET add fields, HGETALL retrieves fields, and renaming keys is done by RENAME.

3. Final Answer:

   Deletes specified fields from a hash -> Option B

4. Quick Check:

   HDEL removes fields = D [OK]

Hint: HDEL always removes fields from a hash [OK]

Common Mistakes:

• Confusing HDEL with HSET (adding fields)
• Thinking HDEL renames keys
• Assuming HDEL retrieves data

2. Which of the following is the correct syntax to remove the field age from a hash named user:1?

easy

A. HDEL age user:1

B. HREMOVE user:1 age

C. DEL user:1 age

D. HDEL user:1 age

Solution

1. Step 1: Recall HDEL syntax

   The correct syntax is HDEL key field [field ...], so the key comes first, then the field(s).

2. Step 2: Check each option

   HDEL user:1 age matches the syntax. HDEL age user:1 reverses key and field. DEL user:1 age uses DEL which deletes keys, not fields. HREMOVE user:1 age uses a non-existent command.

3. Final Answer:

   HDEL user:1 age -> Option D

4. Quick Check:

   HDEL key field = B [OK]

Hint: HDEL syntax: key first, then field(s) [OK]

Common Mistakes:

• Swapping key and field order
• Using DEL instead of HDEL
• Using wrong command names

3. Given the hash user:2 with fields {"name":"Alice", "age":"30", "city":"NY"}, what will be the result of HDEL user:2 age city followed by HGETALL user:2?

medium

A. []

B. ["age", "30", "city", "NY"]

C. ["name", "Alice"]

D. ["name", "Alice", "age", "30"]

Solution

1. Step 1: Understand HDEL effect

   The command HDEL user:2 age city removes the fields age and city from the hash.

2. Step 2: Check remaining fields

   After removal, only the field name with value Alice remains. So HGETALL user:2 returns ["name", "Alice"].

3. Final Answer:

   ["name", "Alice"] -> Option C

4. Quick Check:

   Removed age and city, only name left = A [OK]

Hint: HDEL removes listed fields, others stay [OK]

Common Mistakes:

• Expecting all fields to be deleted
• Confusing HDEL with DEL (which deletes whole key)
• Misreading output format

4. You run the command HDEL user:3 name age but get an error. Which of the following is the most likely cause?

medium

A. The key user:3 holds the wrong kind of value

B. You cannot delete multiple fields at once

C. The command should be HREMOVE instead of HDEL

D. Field names must be numeric

Solution

1. Step 1: Check command validity

   HDEL supports deleting multiple fields and is the correct command; HREMOVE does not exist.

2. Step 2: Consider error causes

   HDEL returns (integer) 0 if the key does not exist (no error). It errors (WRONGTYPE) if the key exists but is not a hash.

3. Step 3: Most common cause

   The most likely cause of an error is that the key user:3 holds the wrong kind of value.

4. Final Answer:

   The key user:3 holds the wrong kind of value -> Option A

5. Quick Check:

   Wrong type causes error = A [OK]

Hint: Check key type (TYPE key) before HDEL [OK]

Common Mistakes:

• Thinking HDEL can't delete multiple fields
• Using wrong command name
• Assuming fields must be numbers

5. You have a Redis hash settings with fields {"theme":"dark", "font":"Arial", "notifications":"on", "volume":"75"}. You want to remove all fields except theme and notifications. Which command(s) should you run?

hard

A. HDEL settings font volume

B. HDEL settings theme notifications

C. DEL settings; HSET settings theme dark notifications on

D. HDEL settings font volume theme notifications

Solution

1. Step 1: Identify fields to remove

   You want to keep theme and notifications, so remove font and volume.

2. Step 2: Choose correct HDEL usage

   HDEL settings font volume deletes only font and volume, preserving the others. HDEL settings theme notifications deletes the fields you want to keep. DEL settings; HSET settings theme dark notifications on deletes the whole hash and recreates it, which is more complex. HDEL settings font volume theme notifications tries to delete all fields including those to keep.

3. Final Answer:

   HDEL settings font volume -> Option A

4. Quick Check:

   Remove unwanted fields only = C [OK]

Hint: HDEL only fields you want removed, keep others [OK]

Common Mistakes:

• Deleting fields to keep
• Deleting entire hash unnecessarily
• Using wrong command to remove fields