EXISTS to check key existence in Redis - Step-by-Step Execution

Concept Flow - EXISTS to check key existence

Start

↓

Check if key exists

↓

Return 1

↓

Return 0

↓

End

The EXISTS command checks if a key is present in the database and returns 1 if yes, 0 if no.

Execution Sample

Redis

EXISTS mykey
EXISTS otherkey

Check if 'mykey' and 'otherkey' exist in Redis, returning 1 or 0.

Execution Table

Step	Command	Key Checked	Exists?	Returned Value
1	EXISTS mykey	mykey	Yes	1
2	EXISTS otherkey	otherkey	No	0

💡 All commands executed; EXISTS returns 1 if key exists, else 0.

Variable Tracker

Key	Initial State	After Step 1	After Step 2
mykey	unknown	exists	exists
otherkey	unknown	unknown	does not exist

Key Moments - 2 Insights

Why does EXISTS return 0 for 'otherkey'?

Because 'otherkey' does not exist in the database at step 2, as shown in the execution_table row 2.

Can EXISTS return values other than 0 or 1?

No, for a single key EXISTS returns only 1 if the key exists or 0 if it does not, as shown in the execution_table.

Visual Quiz - 3 Questions

Look at the execution table, what does EXISTS return for 'mykey' at step 1?

A 0

B 1

C Error

D null

💡 Hint

Check the 'Returned Value' column for step 1 in the execution_table.

At which step does EXISTS find that the key does not exist?

A Step 1

B Both steps

C Step 2

D Neither step

💡 Hint

Look at the 'Exists?' column in the execution_table.

If 'mykey' was deleted before step 1, what would EXISTS return?

A 0

B Error

C 1

D Depends on Redis version

💡 Hint

EXISTS returns 0 if the key does not exist, as shown in the execution_table for 'otherkey'.

Concept Snapshot

EXISTS key
- Checks if a key exists in Redis
- Returns 1 if key exists
- Returns 0 if key does not exist
- Simple way to test key presence

Full Transcript

The EXISTS command in Redis checks if a given key is present in the database. When you run EXISTS with a key name, Redis returns 1 if the key exists and 0 if it does not. For example, if 'mykey' exists, EXISTS mykey returns 1. If 'otherkey' does not exist, EXISTS otherkey returns 0. This command helps you quickly check key presence without retrieving the key's value.