SET and GET commands in Redis - Step-by-Step Execution

Concept Flow - SET and GET commands

Start

↓

SET key value

↓

Store value in key

↓

GET key

↓

Retrieve value

↓

Output value

↓

End

The flow shows storing a value with SET, then retrieving it with GET, and finally outputting the stored value.

Execution Sample

Redis

SET name "Alice"
GET name

Stores the value "Alice" under the key "name" and then retrieves it.

Execution Table

Step	Command	Action	Key	Value Stored	Output
1	SET name "Alice"	Store value	name	"Alice"	OK
2	GET name	Retrieve value	name	"Alice"	"Alice"
3	GET age	Retrieve value	age	null	(nil)

💡 No more commands to execute.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
name	null	"Alice"	"Alice"	"Alice"
age	null	null	null	null

Key Moments - 2 Insights

Why does GET return (nil) when the key does not exist?

GET returns (nil) because the key is not found in the database, as shown in step 3 of the execution_table where 'age' was never set.

What does SET command output after storing a value?

SET outputs 'OK' to confirm the value was stored successfully, as seen in step 1 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output of the SET command at step 1?

A OK

B "Alice"

C (nil)

D Error

💡 Hint

Check the Output column in row for step 1 in execution_table.

At which step does GET return (nil) because the key does not exist?

A Step 1

B Step 2

C Step 3

D None

💡 Hint

Look at the Output column in execution_table for step 3.

If we SET key 'age' to '30' after step 3, what will GET age return?

A (nil)

B "30"

C OK

D Error

💡 Hint

Refer to how SET stores values and GET retrieves them as shown in steps 1 and 2.

Concept Snapshot

SET key value - stores a value under a key
GET key - retrieves the value stored
If key does not exist, GET returns (nil)
SET returns OK on success
GET returns the stored value as a string

Full Transcript

This visual execution shows how Redis commands SET and GET work. First, SET stores a value under a key and returns OK. Then GET retrieves the value for that key. If the key does not exist, GET returns (nil). The variable tracker shows how keys and values change after each command. This helps beginners see exactly what happens step-by-step.

Practice

1. What does the Redis SET command do?

easy

A. Retrieves a value by key

B. Stores a value with a specified key

C. Deletes a key and its value

D. Lists all keys in the database

Solution

1. Step 1: Understand the purpose of SET

   The SET command in Redis is used to save or store a value associated with a key.

2. Step 2: Compare with other commands

   GET retrieves values, DEL deletes keys, and KEYS lists keys, so they are not correct here.

3. Final Answer:

   Stores a value with a specified key -> Option B

4. Quick Check:

   SET saves data = A [OK]

Hint: SET saves data by key, GET fetches it [OK]

Common Mistakes:

• Confusing SET with GET
• Thinking SET deletes data
• Mixing SET with key listing

2. Which of the following is the correct syntax to store the value "hello" with the key "greeting" in Redis?

easy

A. SET greeting hello

B. GET greeting hello

C. SET "greeting" "hello"

D. SET greeting, hello

Solution

1. Step 1: Recall Redis SET syntax

   The correct syntax is SET key value without commas or quotes unless the shell requires them.

2. Step 2: Analyze options

   SET greeting hello matches the correct syntax. GET greeting hello uses GET which is wrong. SET "greeting" "hello" uses quotes unnecessarily. SET greeting, hello uses a comma which is invalid.

3. Final Answer:

   SET greeting hello -> Option A

4. Quick Check:

   SET key value no commas [OK]

Hint: Use SET key value without commas or quotes [OK]

Common Mistakes:

• Using GET instead of SET
• Adding commas between arguments
• Unnecessary quotes around key/value

3. Given the following Redis commands executed in order:

SET color blue
GET color
SET color red
GET color

What will be the output of the two GET commands?

medium

A. "blue" and then "red"

B. "red" and then "blue"

C. "blue" and then "blue"

D. nil and then "red"

Solution

1. Step 1: Analyze the first SET and GET

   The first SET color blue stores "blue". The first GET color returns "blue".

2. Step 2: Analyze the second SET and GET

   The second SET color red overwrites the value with "red". The second GET color returns "red".

3. Final Answer:

   "blue" and then "red" -> Option A

4. Quick Check:

   SET overwrites value, GET returns current value [OK]

Hint: SET overwrites; GET returns latest value [OK]

Common Mistakes:

• Assuming GET returns old value
• Thinking SET appends values
• Expecting nil on first GET

4. You run the command GET username but get a nil response. Which of the following could fix this issue?

medium

A. Run GET username again immediately

B. Use DEL username before GET

C. Restart the Redis server

D. Run SET username "guest" before GET

Solution

1. Step 1: Understand nil response from GET

   Nil means the key "username" does not exist or has no value stored.

2. Step 2: Fix by setting a value

   To get a value, you must first store it using SET username "guest". Other options do not create the key.

3. Final Answer:

   Run SET username "guest" before GET -> Option D

4. Quick Check:

   Nil means no key; SET creates it [OK]

Hint: Nil means missing key; SET it first [OK]

Common Mistakes:

• Retrying GET without SET
• Deleting key before GET
• Restarting server unnecessarily

5. You want to store a user's session token in Redis with key "session:123" and value "abc123token". Which sequence of commands correctly stores and retrieves this token?

hard

A. SET session:123 abc123token
DEL session:123
GET session:123

B. GET session:123
SET session:123 abc123token

C. SET session:123 abc123token
GET session:123

D. SET "session:123" "abc123token"
DEL session:123
GET session:123

Solution

1. Step 1: Store the session token

   Use SET session:123 abc123token to save the token under the key.

2. Step 2: Retrieve the stored token

   Use GET session:123 to fetch the token value.

3. Step 3: Analyze other options

   GET session:123 SET session:123 abc123token tries to GET before SET, so returns nil. Options A and C delete the key before GET, so GET returns nil.

4. Final Answer:

   SET session:123 abc123token GET session:123 -> Option C

5. Quick Check:

   SET then GET returns stored value [OK]

Hint: Always SET before GET to retrieve stored data [OK]

Common Mistakes:

• GET before SET returns nil
• Deleting key before GET
• Using quotes unnecessarily