SUNIONSTORE for storing results in Redis - Mini Project: Build & Apply

Using SUNIONSTORE to Combine Sets in Redis

📖 Scenario: You are managing a Redis database for a social app. You have two sets representing users who liked two different posts. You want to create a new set that stores all users who liked either post.

🎯 Goal: Build a Redis command sequence that creates two sets with exact user IDs, then use SUNIONSTORE to combine these sets into a new set storing all unique users who liked either post.

📋 What You'll Learn

Create a Redis set called post1_likes with members user1, user2, and user3

Create a Redis set called post2_likes with members user3, user4, and user5

Use SUNIONSTORE to store the union of post1_likes and post2_likes into a new set called all_likes

Verify the commands use the exact set names and members as specified

💡 Why This Matters

🌍 Real World

Social media apps often track user interactions like likes or follows using sets in Redis for fast membership checks and combining user groups.

💼 Career

Understanding Redis set operations like SUNIONSTORE is important for backend developers working with caching, real-time analytics, and user data aggregation.

1

Create the first set post1_likes

Use the Redis command SADD to create a set called post1_likes with members user1, user2, and user3.

Redis

# Use SADD to add user1, user2, and user3 to post1_likes
# Your code here

Hint

Use SADD followed by the set name and the members separated by spaces.

2

Create the second set post2_likes

Use the Redis command SADD to create a set called post2_likes with members user3, user4, and user5.

Redis

# Use SADD to add user3, user4, and user5 to post2_likes
# Your code here

Hint

Remember to use the exact set name post2_likes and members.

3

Use SUNIONSTORE to combine the sets

Use the Redis command SUNIONSTORE to create a new set called all_likes that stores the union of post1_likes and post2_likes.

Redis

SADD post1_likes user1 user2 user3
SADD post2_likes user3 user4 user5
# Use SUNIONSTORE to store the union of post1_likes and post2_likes into all_likes
# Your code here

Hint

SUNIONSTORE takes the destination set name first, then the source sets.

4

Verify the union set all_likes

Use the Redis command SMEMBERS to check the members of the set all_likes to confirm it contains all unique users from both sets.

Redis

SADD post1_likes user1 user2 user3
SADD post2_likes user3 user4 user5
SUNIONSTORE all_likes post1_likes post2_likes
# Use SMEMBERS to get all members of all_likes
# Your code here

Hint

SMEMBERS returns all members of a set.

Practice

1. What does the Redis command SUNIONSTORE destination key1 key2 do?

easy

A. It merges the sets stored at key1 and key2 and stores the result in destination.

B. It deletes the sets at key1 and key2 and creates a new set at destination.

C. It returns the union of sets at key1 and key2 without storing it.

D. It copies the set at key1 to destination without merging.

Solution

1. Step 1: Understand SUNIONSTORE purpose

   The command merges multiple sets into one combined set.

2. Step 2: Identify storage behavior

   The result is stored in the specified destination key, overwriting if it exists.

3. Final Answer:

   It merges the sets stored at key1 and key2 and stores the result in destination. -> Option A

4. Quick Check:

   SUNIONSTORE merges and stores = C [OK]

Hint: SUNIONSTORE merges sets and saves result in destination [OK]

Common Mistakes:

• Confusing SUNIONSTORE with SUNION (which does not store)
• Thinking it deletes original sets
• Assuming it copies only one set

2. Which of the following is the correct syntax to store the union of sets setA and setB into resultSet using Redis?

easy

A. SUNIONSTORE setA setB resultSet

B. SUNIONSTORE resultSet setA setB

C. SUNION resultSet setA setB

D. SUNIONSTORE setA resultSet setB

Solution

1. Step 1: Recall SUNIONSTORE syntax

   The correct syntax is SUNIONSTORE destination key1 key2 ....

2. Step 2: Match keys to syntax

   Here, resultSet is destination, followed by sets setA and setB.

3. Final Answer:

   SUNIONSTORE resultSet setA setB -> Option B

4. Quick Check:

   Destination first, then sets = A [OK]

Hint: Destination key comes first in SUNIONSTORE command [OK]

Common Mistakes:

• Placing destination key last
• Using SUNION instead of SUNIONSTORE for storing
• Mixing order of keys

3. You have three sets:
setX = {1, 2, 3}
setY = {3, 4, 5}
setZ = {5, 6}
You want to create a new set combined that contains the union of all three sets. Which command correctly does this and stores the result in combined?

easy

A. SUNIONSTORE combined setX setY setZ

B. SUNIONSTORE combined setX setY

C. SUNION combined setX setY setZ

D. SUNIONSTORE setX setY setZ combined

Solution

1. Step 1: Recall SUNIONSTORE usage for multiple sets

   SUNIONSTORE can take multiple keys after the destination key.

2. Step 2: Verify correct order and keys

   Destination is combined, followed by setX, setY, and setZ.

3. Final Answer:

   SUNIONSTORE combined setX setY setZ -> Option A

4. Quick Check:

   Destination first, then all sets = B [OK]

Hint: List all sets after destination in SUNIONSTORE [OK]

Common Mistakes:

• Using SUNION instead of SUNIONSTORE to store
• Placing destination key last
• Omitting one or more sets

4. Given the following Redis sets:
SADD set1 a b c
SADD set2 b c d
What will be the members of resultSet after executing SUNIONSTORE resultSet set1 set2?

medium

A. ["a", "d"]

B. ["a", "b", "c"]

C. ["a", "b", "c", "d"]

D. ["b", "c", "d"]

Solution

1. Step 1: Identify members of each set

   set1 has {a, b, c}, set2 has {b, c, d}.

2. Step 2: Compute union of both sets

   The union is {a, b, c, d} combining all unique members.

3. Final Answer:

   ["a", "b", "c", "d"] -> Option C

4. Quick Check:

   Union of set1 and set2 = ["a", "b", "c", "d"] [OK]

Hint: Union combines all unique elements from both sets [OK]

Common Mistakes:

• Forgetting to include all unique elements
• Confusing union with intersection
• Assuming order matters in sets

5. You run the command SUNIONSTORE result set1 set2 but get an error. What is the most likely cause?

medium

A. SUNIONSTORE does not accept more than two sets.

B. You forgot to specify the destination key.

C. The sets set1 and set2 do not exist.

D. The key result already exists as a string, not a set.

Solution

1. Step 1: Understand data type requirements

   SUNIONSTORE requires the destination key to be a set or not exist.

2. Step 2: Identify error cause

   If result exists as a string or other type, Redis throws a type error.

3. Final Answer:

   The key result already exists as a string, not a set. -> Option D

4. Quick Check:

   Destination key type mismatch causes error [OK]

Hint: Destination key must be a set or not exist [OK]

Common Mistakes:

• Assuming missing keys cause errors (they don't)
• Thinking SUNIONSTORE limits number of sets
• Ignoring key data types