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A **Set** in Redis is an **unordered set of unique string values**.
Every element is a Redis String, but with no duplicates and no preserved order.

### Main properties

- Every element is unique (duplicates aren't stored).
- Element order isn't fixed.
- Fast set operations are supported, union, intersection, difference.
- It fits storing unique values: IDs, tags, users, and so on.

### Example

```bash
SADD users "anna" "ivan" "maria"
SADD users "ivan"
SMEMBERS users
```

Result:

```javascript
1) "anna"
2) "ivan"
3) "maria"
```

The second `"ivan"` add is ignored, the element already exists.

### Main commands

| Command | Description |
|---|---|
| `SADD key value [value ...]` | add element(s) |
| `SREM key value [value ...]` | remove element(s) |
| `SMEMBERS key` | get every element in the set |
| `SISMEMBER key value` | check whether a value exists |
| `SCARD key` | the number of elements |
| `SPOP key [count]` | remove and return a random element |
| `SRANDMEMBER key [count]` | get a random element without removing it |
| `SUNION key [key ...]` | union of sets |
| `SINTER key [key ...]` | intersection of sets |
| `SDIFF key [key ...]` | difference of sets |

### A union and intersection example

```bash
SADD groupA "anna" "maria"
SADD groupB "maria" "ivan"
SUNION groupA groupB
SINTER groupA groupB
```

Result:

```javascript
SUNION → "anna", "maria", "ivan"
SINTER → "maria"
```

### Internal structure

Redis stores sets in two formats:

1. **intset**, if every element is an integer and there aren't many (up to 512 elements by default).
2. **hashtable**, if there are more elements or they include strings.

intset saves memory, hashtable provides fast insertion and lookup.

### Uses

- Storing unique user IDs or tokens.
- Lists of subscribers or members with no duplicates.
- Fast membership checks (`SISMEMBER`).
- Recommendation systems via set intersection (`SINTER`).
- Counting unique visits or actions (`SCARD`).

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