---
{
    "title": "bitmap_union",
    "language": "zh-CN"
}
---

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# bitmap_union
## description

聚合函数，用于计算分组后的 bitmap 并集。常见使用场景如：计算PV，UV。

### Syntax

`BITMAP BITMAP_UNION(BITMAP value)`

输入一组 bitmap 值，求这一组 bitmap 值的并集，并返回。

## example

```
mysql> select page_id, bitmap_union(user_id) from table group by page_id;
```

和 bitmap_count 函数组合使用可以求得网页的 PV 数据

```
mysql> select page_id, bitmap_count(bitmap_union(user_id)) from table group by page_id;
```

当 user_id 字段为 int 时，上面查询语义等同于

```
mysql> select page_id, count(distinct user_id) from table group by page_id;
```

## keyword

    BITMAP_UNION, BITMAP