EXISTS to check key existence in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Redis command EXISTS do?

The EXISTS command checks if a given key exists in the Redis database. It returns 1 if the key exists, and 0 if it does not.

intermediate

How do you check multiple keys at once with EXISTS in Redis?

You can pass multiple keys to EXISTS. It returns the number of keys that exist among those given.

beginner

What is the return type of the EXISTS command in Redis?

It returns an integer: 1 if the key exists, 0 if it does not. When checking multiple keys, it returns the count of existing keys.

beginner

If you run EXISTS mykey and the key does not exist, what will Redis return?

Redis will return 0, indicating the key mykey does not exist.

beginner

Why is using EXISTS useful before performing operations on a key?

It helps avoid errors or unnecessary work by confirming the key is present before trying to read or modify it.

What does the Redis command EXISTS mykey return if mykey is present?

A 1

B 0

C null

D error

EXISTS returns 1 if the key exists.

How does EXISTS behave when checking multiple keys?

A Returns a list of keys that exist

B Returns 1 if all keys exist

C Returns 0 if any key is missing

D Returns the count of keys that exist

EXISTS returns the number of keys that exist among those checked.

Which of these is a valid use of EXISTS?

A EXISTS(key1, key2, key3)

B EXISTS key1, key2, key3

C EXISTS key1 key2 key3

D EXISTS [key1 key2 key3]

Redis EXISTS accepts multiple keys separated by spaces.

If EXISTS returns 0, what does it mean?

A No keys exist

B Command failed

C At least one key exists

D All keys exist

A return of 0 means none of the keys exist.

Why might you check key existence with EXISTS before deleting a key?

A To avoid deleting a key twice

B To confirm the key is present before deleting

C To rename the key

D To get the key's value

Checking existence helps avoid errors or unnecessary commands.

Explain how the Redis EXISTS command works and what it returns when checking one or multiple keys.

Think about how you check if something is there or not.

Describe a practical situation where using EXISTS before another Redis command is helpful.

Imagine you want to be sure before you act.

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