Working with Redis Sets

Redis supports several collection types, and one of the most useful is the Set. A Set is an unordered collection of unique values, similar to a Python set or a mathematical set.

Sets are often used to group related keys — for example, all products by brand, or all users with a specific role.

Example

 SADD brand:Dell product:1005
 SADD brand:Dell product:1006
 SADD brand:Apple product:2001

Redis automatically creates the sets if they don’t exist.

You can now list all products that belong to Dell:

 SMEMBERS brand:Dell

Output:

 1) "product:1005"
 2) "product:1006"

### Common Set Commands

Command	Description	Example	Result
`SADD key value`	Add one or more elements to a set	`SADD tag:laptop`	`1` (new element added)
`SMEMBERS key`	Get all elements in the set	`SMEMBERS tag:laptop`	`"product:1005"`
`SISMEMBER key value`	Check if an element exists	`SISMEMBER tag:laptop product:1005`	`1` (true)
`SREM key value`	Remove an element from the set	`SREM tag:laptop product:1005`	`1`
`SUNION key1 key2`	Return the union of two sets	`SUNION tag:laptop tag:tablet`	All unique elements from both sets
`SINTER key1 key2`	Return intersection (common elements)	`SINTER tag:electronics tag:laptop`	Common product IDs
`SDIFF key1 key2`	Return difference	`SDIFF tag:electronics tag:laptop`	Items in first but not second set

Visualizing Sets in Redis

Redis sets are perfect for modeling simple relationships — such as grouping products by brand or tagging items with categories.
Because sets can overlap (the same element can exist in multiple sets), they are similar to mathematical sets in discrete mathematics.

Example 1 – A single set

graph TD
    subgraph brand:Dell
        P1["product:1005"]
        P2["product:1006"]
    end

This shows one Redis set called brand:Dell, containing two products. Each product key is unique and belongs to that brand.

Example 2 – A overlapping set

graph TD
    subgraph brand:Dell
        D1["product:1005"]
        D2["product:1006"]
        D3["product:1007"]
    end

    subgraph tag:Electronics
        E1["product:1005"]
        E2["product:2001"]
        E3["product:3002"]
    end

Here, the key product:1005 appears in both sets. This overlap represents a product that belongs to brand Dell and is tagged as “Electronics.” Such overlaps are the foundation for set operations like intersections and unions.

Example 3 - Set operations

The intersection

graph TD
    subgraph A["Set A (brand:Dell)"]
        A1["product:1005"]
        A2["product:1006"]
        A3["product:1007"]
    end

    subgraph B["Set B (tag:Electronics)"]
        B1["product:1005"]
        B2["product:2001"]
        B3["product:3002"]
    end

    subgraph C["Result of SINTER(A, B)"]
        C1["product:1005"]
    end

    A -- "SINTER" --> C
    B -- "SINTER" --> C

The intersection returns all elements which occur in both set A and in set B.

The union

graph TD
    subgraph A["Set A (brand:Dell)"]
        A1["product:1005"]
        A2["product:1006"]
        A3["product:1007"]
    end

    subgraph B["Set B (tag:Electronics)"]
        B1["product:1005"]
        B2["product:2001"]
        B3["product:3002"]
    end

    subgraph C["Result of SUNION(A, B)"]
        C1["product:1005"]
        C2["product:1006"]
        C3["product:1007"]
        C4["product:2001"]
        C5["product:3002"]
    end

    A -- "SUNION" --> C
    B -- "SUNION" --> C

The union returns all unique elements that are present in set A, set B, or both.

The difference

graph TD
    subgraph A["Set A (brand:Dell)"]
        A1["product:1005"]
        A2["product:1006"]
        A3["product:1007"]
    end

    subgraph B["Set B (tag:Electronics)"]
        B1["product:1005"]
        B2["product:2001"]
        B3["product:3002"]
    end

    subgraph C["Result of SDIFF(A, B)"]
        C1["product:1006"]
        C2["product:1007"]        
    end

    A -- "SDIFF" --> C

The difference returns the elements that exist only in set A and are not shared with set B.

In this visualization, there is only an arrow from set A to set C, because only set A serves as the data source. Set B is used for comparison only.

### Why Sets Are Useful

Sets are great for modeling relationships or categories, for example:

Products grouped by brand (brand:Dell, brand:Apple)
Users grouped by role (role:admin, role:student)
Tags or labels (tag:featured, tag:sale)
Active sessions or online users

Example:

 SADD role:admin user:101 user:102
 SADD role:student user:201 user:202
 SMEMBERS role:admin

Summary

A Redis Set is an unordered collection of unique elements.
Use SADD to add, SMEMBERS to list, and SISMEMBER to check membership.
Sets are ideal for grouping related items or creating simple lookup relationships.
They are often used to simulate indexes — e.g., all products that belong to a specific category or brand.

Practice Questions

🧠 1. We have two sets:

Set A = { book, laptop, phone }
Set B = { tablet, phone, camera }

What is the intersection of A and B?

Click to reveal answer

SINTER A B \ → { phone } The intersection returns only elements that appear in **both sets**.

🧠 2. Union of two sets

Set A = { pen, notebook }
Set B = { notebook, charger, mouse }

What is the union of A and B?

Click to reveal answer

SUNION A B \ → { pen, notebook, charger, mouse } The union combines all the elements from **both sets**.

🧠 3. Difference between two sets

Set A = { apple, banana, kiwi, orange }
Set B = { kiwi, mango }

What is the difference (A - B)?

Click to reveal answer

SDIFF A B \ → { apple, banana, orange } The difference return the elements of set A which are not in set B.