APPEND for string concatenation in Redis

Introduction

APPEND lets you add more text to the end of a string stored in Redis without replacing the original text.

You want to build a message step-by-step in Redis.

You need to add new data to a log stored as a string.

You want to update a user's status message by adding more details.

You are collecting parts of a report and want to join them in Redis.

You want to keep adding notes to a string without losing old notes.

Syntax

Redis

APPEND key value

If the key does not exist, APPEND creates it with the given value.

APPEND returns the new length of the string after adding the value.

Examples

Adds "Hello" to the key 'greeting'. If 'greeting' does not exist, it creates it.

Redis

APPEND greeting "Hello"

Adds ", world!" to the existing 'greeting' string.

Redis

APPEND greeting ", world!"

Retrieves the full concatenated string stored in 'greeting'.

Redis

GET greeting

Sample Program

First, it adds "Hi" to 'message'. Then it adds ", how are you?" to the same key. Finally, it gets the full message.

Redis

APPEND message "Hi"
APPEND message ", how are you?"
GET message

Important Notes

APPEND works only with string values in Redis.

If you use APPEND on a key holding a different data type, Redis will return an error.

You can use APPEND multiple times to build a longer string gradually.

Summary

APPEND adds text to the end of a string in Redis without replacing it.

If the key does not exist, APPEND creates it with the new text.

APPEND returns the new length of the string after adding the text.

Practice

1. What does the Redis APPEND command do when used on a string key?

easy

A. Returns the current string value without changes.

B. Replaces the entire string value with the new text.

C. Deletes the string key from the database.

D. Adds the given text to the end of the existing string value.

Solution

1. Step 1: Understand the APPEND command purpose

   The APPEND command adds new text to the end of an existing string without removing the old text.

2. Step 2: Compare options with command behavior

   Only Adds the given text to the end of the existing string value. correctly describes adding text to the end, not replacing or deleting.

3. Final Answer:

   Adds the given text to the end of the existing string value. -> Option D

4. Quick Check:

   APPEND adds text to end [OK]

Hint: APPEND never replaces, it always adds at the end [OK]

Common Mistakes:

• Thinking APPEND replaces the string
• Confusing APPEND with DELETE command
• Assuming APPEND returns the string value

2. Which of the following is the correct syntax to append the string 'world' to the key 'greeting' in Redis?

easy

A. APPEND greeting world

B. APPEND 'greeting' world

C. APPEND greeting 'world'

D. APPEND 'greeting' 'world'

Solution

1. Step 1: Recall Redis command syntax

   Redis commands use the format: COMMAND key value without quotes unless the value contains spaces.

2. Step 2: Identify correct option

   APPEND greeting world uses correct syntax: APPEND greeting world (no quotes needed here).

3. Final Answer:

   APPEND greeting world -> Option A

4. Quick Check:

   Redis commands: APPEND key value [OK]

Hint: Use APPEND key value without quotes unless spaces exist [OK]

Common Mistakes:

• Adding quotes unnecessarily around key or value
• Swapping key and value order
• Using commas or other separators

3. Given the following Redis commands executed in order:

SET key1 "Hello"
APPEND key1 " World"
APPEND key1 "!"

What will be the final length returned by the last APPEND command?

medium

A. 12

B. 13

C. 6

D. 11

Solution

1. Step 1: Calculate initial string length

   SET key1 "Hello" sets string length to 5 characters.

2. Step 2: Add " World" with APPEND

   " World" has 6 characters (including space), new length = 5 + 6 = 11.

3. Step 3: Add "!" with APPEND

   Adding "!" (1 character) makes length 11 + 1 = 12, but note quotes are not stored.

4. Step 4: Recount carefully

   "Hello" (5) + " World" (6) + "!" (1) = 12 characters total.

5. Final Answer:

   12 -> Option A

6. Quick Check:

   5 + 6 + 1 = 12 [OK]

Hint: Count characters added stepwise, including spaces [OK]

Common Mistakes:

• Counting quotes as characters
• Forgetting spaces in appended strings
• Adding lengths incorrectly

4. You run the command APPEND mykey "text" but get an error: "WRONGTYPE Operation against a key holding the wrong kind of value". What is the most likely cause?

medium

A. The key 'mykey' does not exist yet.

B. The key 'mykey' holds a non-string value like a list or set.

C. The APPEND command syntax is incorrect.

D. Redis server is down.

Solution

1. Step 1: Understand the error message

   "WRONGTYPE" means the key exists but is not a string type.

2. Step 2: Analyze possible causes

   If the key held a string or did not exist, APPEND would work or create it. Syntax error or server down would cause different errors.

3. Final Answer:

   The key 'mykey' holds a non-string value like a list or set. -> Option B

4. Quick Check:

   WRONGTYPE means key type mismatch [OK]

Hint: WRONGTYPE means key is not a string [OK]

Common Mistakes:

• Assuming key absence causes WRONGTYPE
• Blaming syntax for WRONGTYPE error
• Ignoring key data type

5. You want to build a Redis string key 'log' that starts empty and appends multiple messages separated by commas. Which sequence of commands correctly achieves this without leading commas?

hard

A. APPEND log ",message1" APPEND log ",message2" APPEND log ",message3"

B. APPEND log "message1" APPEND log "message2" APPEND log "message3"

C. SET log "message1" APPEND log ",message2" APPEND log ",message3"

D. SET log "message1," APPEND log "message2," APPEND log "message3,"

Solution

1. Step 1: Initialize the key without leading comma

   Use SET to start 'log' with "message1" (no comma at start).

2. Step 2: Append subsequent messages with leading commas

   Use APPEND to add ",message2" and ",message3" to separate messages properly.

3. Step 3: Verify no leading comma and correct separators

   SET log "message1" APPEND log ",message2" APPEND log ",message3" correctly avoids leading comma and separates messages with commas.

4. Final Answer:

   SET log "message1" APPEND log ",message2" APPEND log ",message3" -> Option C

5. Quick Check:

   Start with SET, then APPEND with commas [OK]

Hint: Start with SET, then APPEND with commas to separate [OK]

Common Mistakes:

• Starting with APPEND adding commas first
• Appending without commas causing merged words
• Adding trailing commas unnecessarily