SUNION, SINTER, SDIFF set operations in Redis

Introduction

These commands help you combine or compare groups of unique items stored in sets, making it easy to find common, different, or all items between them.

You want to find all unique friends from two different friend lists.

You need to find common interests between two groups of users.

You want to see which items are in one list but not in another.

You want to merge tags from multiple articles without duplicates.

You want to filter out items that appear in both sets.

Syntax

Redis

SUNION key1 [key2 ...]
SINTER key1 [key2 ...]
SDIFF key1 [key2 ...]

Each command works with one or more set keys stored in Redis.

The result is a set of unique items based on the operation.

Examples

Returns all unique members from both setA and setB combined.

Redis

SUNION setA setB

Returns only the members that exist in both setA and setB.

Redis

SINTER setA setB

Returns members in setA that are not in setB.

Redis

SDIFF setA setB

Sample Program

First, we add fruits to two sets. Then we find all unique fruits (union), common fruits (intersection), and fruits only in setA (difference).

Redis

SADD setA apple banana cherry
SADD setB banana cherry date
SUNION setA setB
SINTER setA setB
SDIFF setA setB

Important Notes

SUNION combines all unique elements from the given sets.

SINTER finds only elements present in every set.

SDIFF returns elements from the first set that are missing in the others.

Summary

SUNION merges sets and removes duplicates.

SINTER finds common elements between sets.

SDIFF finds elements unique to the first set.

Practice

1. What does the Redis command SUNION do when applied to multiple sets?

easy

A. It returns all unique elements from all given sets combined.

B. It returns only the elements common to all given sets.

C. It returns elements unique to the first set only.

D. It deletes all the given sets from the database.

Solution

1. Step 1: Understand SUNION operation

   The SUNION command merges all elements from the given sets and removes duplicates.

2. Step 2: Compare with other set operations

   SINTER returns only common elements, SDIFF returns unique elements from the first set, so SUNION is the only one that merges all uniquely.

3. Final Answer:

   It returns all unique elements from all given sets combined. -> Option A

4. Quick Check:

   SUNION = all unique elements combined [OK]

Hint: SUNION combines all sets without duplicates [OK]

Common Mistakes:

• Confusing SUNION with SINTER (common elements)
• Thinking SUNION returns only first set elements
• Assuming SUNION deletes sets

2. Which of the following is the correct syntax to find the intersection of two sets set1 and set2 in Redis?

easy

A. SINTER set1 set2

B. SUNION set1 set2

C. SDIFF set1 set2

D. SINTERSECTION set1 set2

Solution

1. Step 1: Recall Redis command for intersection

   The correct Redis command to find common elements is SINTER.

2. Step 2: Check syntax correctness

   The syntax is SINTER key1 key2 .... SINTER set1 set2 matches this exactly.

3. Final Answer:

   SINTER set1 set2 -> Option A

4. Quick Check:

   SINTER syntax = SINTER keys [OK]

Hint: Use SINTER for intersection syntax [OK]

Common Mistakes:

• Using SUNION instead of SINTER
• Using non-existent command SINTERSECTION
• Confusing SDIFF with intersection

3. Given the following Redis sets:
set1 = {"apple", "banana", "cherry"}
set2 = {"banana", "date", "fig"}
What is the result of SINTER set1 set2?

medium

A. ["apple", "banana", "cherry", "date", "fig"]

B. ["banana"]

C. ["apple", "cherry"]

D. ["date", "fig"]

Solution

1. Step 1: Identify common elements between set1 and set2

   Elements in set1: apple, banana, cherry; set2: banana, date, fig. The only common element is banana.

2. Step 2: Understand SINTER output

   SINTER returns only elements present in all sets, so the result is ["banana"].

3. Final Answer:

   ["banana"] -> Option B

4. Quick Check:

   SINTER common elements = ["banana"] [OK]

Hint: SINTER returns only common elements [OK]

Common Mistakes:

• Listing all elements instead of common ones
• Confusing SINTER with SUNION or SDIFF
• Including elements unique to one set

4. You run the command SDIFF set1 set2 but get an unexpected empty result. Given:
set1 = {"cat", "dog"}
set2 = {"cat", "dog", "mouse"}
What is the likely reason?

medium

A. set1 is empty, so SDIFF returns empty.

B. SDIFF syntax requires only one set argument.

C. Redis does not support SDIFF with these sets.

D. All elements of set1 are also in set2, so no unique elements remain.

Solution

1. Step 1: Understand SDIFF behavior

   SDIFF returns elements in the first set that are not in the other sets.

2. Step 2: Analyze given sets

   set1 has "cat" and "dog"; set2 has "cat", "dog", and "mouse". Since all set1 elements are in set2, SDIFF returns an empty set.

3. Final Answer:

   All elements of set1 are also in set2, so no unique elements remain. -> Option D

4. Quick Check:

   SDIFF unique first set elements = empty [OK]

Hint: SDIFF returns only unique first set elements [OK]

Common Mistakes:

• Assuming SDIFF requires only one set
• Thinking SDIFF returns common elements
• Believing Redis lacks SDIFF support

5. You have three Redis sets:
setA = {"red", "blue", "green"}
setB = {"blue", "yellow"}
setC = {"green", "yellow", "black"}
Which command returns elements that are in setA but not in setB or setC?

hard

A. SUNION setA setB setC

B. SINTER setA setB setC

C. SDIFF setA setB setC

D. SDIFF setB setA setC

Solution

1. Step 1: Understand SDIFF with multiple sets

   SDIFF returns elements in the first set that are not in any of the other sets.

2. Step 2: Apply SDIFF to given sets

   Elements in setA: red, blue, green. Elements in setB and setC combined: blue, yellow, green, black. Elements unique to setA are only "red".

3. Step 3: Verify command correctness

   SDIFF setA setB setC uses SDIFF with setA first, then setB and setC, which matches the requirement.

4. Final Answer:

   SDIFF setA setB setC -> Option C

5. Quick Check:

   SDIFF first set minus others = unique elements [OK]

Hint: Use SDIFF with first set then others to find unique elements [OK]

Common Mistakes:

• Using SINTER or SUNION instead of SDIFF
• Reversing set order in SDIFF
• Expecting SDIFF to return common elements