EXISTS to check key existence in Redis

Introduction

We use EXISTS to quickly check if a key is present in the database without retrieving its value.

Before updating a key, to make sure it exists.

To check if a user session key is active.

To verify if a cache entry is available before fetching data.

To conditionally perform actions only if a key is stored.

To avoid errors when deleting keys that might not exist.

Syntax

Redis

EXISTS key [key ...]

You can check one or multiple keys at once by listing them.

The command returns the number of keys that exist among the given keys.

Examples

Check if the key 'user:1000' exists.

Redis

EXISTS user:1000

Check if both 'session:1234' and 'cart:5678' keys exist; returns how many exist.

Redis

EXISTS session:1234 cart:5678

Sample Program

First, we set a key 'user:1' with value 'Alice'. Then we check if 'user:1' exists (should be 1). Next, check 'user:2' which was not set (should be 0). Finally, check both keys together; only 'user:1' exists, so result is 1.

Redis

SET user:1 "Alice"
EXISTS user:1
EXISTS user:2
EXISTS user:1 user:2

Important Notes

EXISTS returns an integer: the count of keys found, not a boolean true/false.

If you check multiple keys, the result tells how many exist, not which ones.

Use EXISTS to avoid errors when working with keys that might not be present.

Summary

EXISTS checks if one or more keys are in the database.

It returns how many of the given keys exist.

Useful to confirm presence before reading or modifying 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