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A **Sorted Set (ZSet)** in Redis is a **set of unique elements**, where **each element is tied to a numeric value, a "score"**.
These elements are automatically kept **sorted by score**.

### Main properties

- Every element is unique, just like in a regular Set.
- Every element has a **score**, a floating-point number.
- Elements are **always ordered** by score (ascending).
- Ranges can be fetched by position or by score.

### Example

```bash
ZADD rating 100 "anna" 200 "ivan" 150 "maria"
ZRANGE rating 0 -1 WITHSCORES
```

Result:

```javascript
1) "anna" (100)
2) "maria" (150)
3) "ivan" (200)
```

Elements are automatically sorted by score.

### Main commands

| Command | Description |
|---|---|
| `ZADD key score member [score member ...]` | add elements with scores |
| `ZRANGE key start stop [WITHSCORES]` | get elements by position (ascending) |
| `ZREVRANGE key start stop [WITHSCORES]` | the same, but descending |
| `ZRANGEBYSCORE key min max [WITHSCORES]` | get elements within a score range |
| `ZREM key member [member ...]` | remove element(s) |
| `ZSCORE key member` | get an element's score |
| `ZCARD key` | the number of elements |
| `ZCOUNT key min max` | the number of elements within a score range |
| `ZRANK key member` | an element's ascending position |
| `ZREVRANK key member` | an element's descending position |
| `ZINCRBY key increment member` | increment an element's score |

### A leaderboard example

```bash
ZADD leaderboard 300 "player1"
ZADD leaderboard 500 "player2"
ZADD leaderboard 450 "player3"
ZREVRANGE leaderboard 0 2 WITHSCORES
```

Result:

```javascript
1) "player2" (500)
2) "player3" (450)
3) "player1" (300)
```

This is how **leaderboards, rankings, points, and task priorities** get stored.

### A score range

```bash
ZRANGEBYSCORE leaderboard 400 600
```

Result:

```javascript
"player2"
"player3"
```

### Internal structure

A ZSet is implemented as a combination of:

1. A **Hash**, for fast lookup of a score by an element's name.
2. A **Skip list**, for keeping elements sorted.

That's what gives it **O(log N)** complexity for insertion, lookup, and deletion.

### Uses

- Leaderboards, rankings.
- Priority queues.
- Event timestamps (time series).
- Tracking user activity.
- Task lists by priority or deadline.

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