EXISTS to check key existence in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Check if a single key exists

Given the Redis database contains keys: user:1, user:2, and session:1, what is the output of the command EXISTS user:1?

Redis

EXISTS user:1

A 1

B 0

C Error: key not found

D null

💡 Hint

EXISTS returns 1 if the key exists, otherwise 0.

The EXISTS command returns 1 if the key is present in the database. Since user:1 exists, the output is 1.

❓ query_result

intermediate

Check multiple keys existence count

What does the command EXISTS user:1 user:3 session:1 return if only user:1 and session:1 exist in the Redis database?

Redis

EXISTS user:1 user:3 session:1

A 3

B 1

C 2

D 0

💡 Hint

EXISTS returns the number of keys that exist among the given keys.

Only user:1 and session:1 exist, so EXISTS returns 2.

📝 Syntax

advanced

Identify the syntax error in EXISTS command

Which option shows an invalid syntax for the EXISTS command in Redis?

A EXISTS user:3

B EXISTS (user:1)

C EXISTS session:1

D EXISTS user:1 user:2

💡 Hint

EXISTS command does not use parentheses around keys.

Parentheses are not valid syntax in Redis commands. Option B uses parentheses, causing a syntax error.

❓ optimization

advanced

Optimizing key existence checks

You want to check if any of the keys user:1, user:2, user:3 exist. Which command is the most efficient to get the count of existing keys?

A EXISTS user:1; EXISTS user:2; EXISTS user:3

B KEYS user:*

C GET user:1 user:2 user:3

D EXISTS user:1 user:2 user:3

💡 Hint

EXISTS can take multiple keys at once and returns the count of existing keys.

Using EXISTS with multiple keys in one command is efficient and returns the count directly. Option D sends multiple commands, which is less efficient.

🧠 Conceptual

expert

Behavior of EXISTS with expired keys

If a key named session:temp has expired and been deleted by Redis, what will EXISTS session:temp return?

Redis

EXISTS session:temp

A 0

B 1

C Error: key expired

D null

💡 Hint

Expired keys are automatically removed and do not exist anymore.

Once a key expires, it is deleted and no longer exists. EXISTS returns 0 for such keys.

Practice

1. What does the Redis command EXISTS key do?

easy

A. Checks if the specified key exists and returns 1 if yes, 0 if no

B. Deletes the specified key from the database

C. Returns the value stored at the specified key

D. Sets a new key with a default value

Solution

1. Step 1: Understand the purpose of EXISTS

   The EXISTS command is used to check if a key is present in the Redis database.

2. Step 2: Know the return values

   It returns 1 if the key exists, and 0 if it does not.

3. Final Answer:

   Checks if the specified key exists and returns 1 if yes, 0 if no -> Option A

4. Quick Check:

   EXISTS returns 1 or 0 for key presence [OK]

Hint: EXISTS returns 1 if key is found, else 0 [OK]

Common Mistakes:

• Thinking EXISTS deletes keys
• Assuming EXISTS returns the key's value
• Confusing EXISTS with SET command

2. Which of the following is the correct syntax to check if a key named user:1 exists in Redis?

easy

A. EXIST user:1

B. EXISTS user:1

C. CHECK user:1

D. HASKEY user:1

Solution

1. Step 1: Identify the correct Redis command

   The correct command to check key existence is EXISTS.

2. Step 2: Verify the syntax

   The syntax is EXISTS keyname, so EXISTS user:1 is correct.

3. Final Answer:

   EXISTS user:1 -> Option B

4. Quick Check:

   EXISTS keyname is correct syntax [OK]

Hint: EXISTS is the exact command to check keys [OK]

Common Mistakes:

• Using non-existent commands like EXIST or HASKEY
• Misspelling EXISTS
• Adding extra keywords after EXISTS

3. Given the following Redis keys: user:1 and user:2 exist, but user:3 does not. What is the result of EXISTS user:1 user:3 user:2?

medium

A. 2

B. 1

C. 3

D. 0

Solution

1. Step 1: Understand EXISTS with multiple keys

   When given multiple keys, EXISTS returns the count of keys that exist.

2. Step 2: Count existing keys in the list

   Keys user:1 and user:2 exist, but user:3 does not, so count is 2.

3. Final Answer:

   2 -> Option A

4. Quick Check:

   EXISTS counts existing keys = 2 [OK]

Hint: EXISTS with multiple keys returns count of existing keys [OK]

Common Mistakes:

• Assuming EXISTS returns 1 if any key exists
• Thinking it returns all keys found
• Confusing count with boolean result

4. You run the command EXISTS without any key arguments in Redis. What happens?

medium

A. Returns 0 because no keys were specified

B. Deletes all keys in the database

C. Returns an error due to missing key argument

D. Returns 1 by default

Solution

1. Step 1: Check command requirements

   The EXISTS command requires at least one key argument to check.

2. Step 2: Understand Redis behavior on missing arguments

   If no keys are provided, Redis returns a syntax error indicating missing arguments.

3. Final Answer:

   Returns an error due to missing key argument -> Option C

4. Quick Check:

   EXISTS needs keys, else error [OK]

Hint: EXISTS must have keys; no keys causes error [OK]

Common Mistakes:

• Assuming it returns 0 with no keys
• Thinking it returns 1 by default
• Believing it deletes keys without arguments

5. You want to check if any of the keys session:1, session:2, or session:3 exist, and if none exist, set session:1 with a value. Which Redis command sequence correctly uses EXISTS to do this safely?

hard

A. EXISTS session:1 session:2 session:3\nIF 1 THEN SET session:1 'active' END

B. IF EXISTS session:1 session:2 session:3 == 0 THEN SET session:1 'active' END

C. EXISTS session:1 session:2 session:3\nIF EXISTS == 0 THEN SET session:1 'active' END

D. EXISTS session:1 session:2 session:3\nIF 0 THEN SET session:1 'active' END

Solution

1. Step 1: Use EXISTS to check multiple keys

   EXISTS returns the count of existing keys among the given list.

2. Step 2: Use conditional logic to set key if none exist

   If EXISTS returns 0, it means none of the keys exist, so we can safely set session:1.

3. Step 3: Choose the correct syntax for conditional check

   EXISTS session:1 session:2 session:3\nIF 0 THEN SET session:1 'active' END shows the correct sequence: run EXISTS, then if result is 0, set the key.

4. Final Answer:

   EXISTS session:1 session:2 session:3\nIF 0 THEN SET session:1 'active' END -> Option D

5. Quick Check:

   EXISTS count 0 means no keys, then set key [OK]

Hint: Check EXISTS count == 0 before setting key [OK]

Common Mistakes:

• Using incorrect conditional syntax
• Not checking all keys at once
• Setting key without checking existence