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# sumSimpleState

> sumSimpleState 组合器使用示例

<div id="description">
  ## 描述
</div>

[`SimpleState`](/zh/reference/functions/aggregate-functions/combinators#-simplestate) 组合器可应用于 [`sum`](/zh/reference/functions/aggregate-functions/sum)
函数，以返回所有输入值之和。返回结果的类型为
[`SimpleAggregateFunction`](/zh/reference/data-types/simpleaggregatefunction)。

<div id="example-usage">
  ## 示例用法
</div>

<div id="tracking-post-votes">
  ### 跟踪顶票和踩票
</div>

来看一个使用表来跟踪帖子投票情况的实际示例。
对于每篇帖子，我们希望维护顶票数、踩票数以及
总分的累计值。对于这种用例，使用带有 sum 的 `SimpleAggregateFunction` 类型非常合适，
因为我们只需要存储这些累计值，而不必存储聚合的完整状态。
因此，它会更快，而且无需合并部分聚合状态。

首先，我们为原始数据创建一个表：

```sql title="Query" theme={null}
CREATE TABLE raw_votes
(
    post_id UInt32,
    vote_type Enum8('upvote' = 1, 'downvote' = -1)
)
ENGINE = MergeTree()
ORDER BY post_id;
```

接下来，创建一个用于存储聚合数据的目标表：

```sql theme={null}
CREATE TABLE vote_aggregates
(
    post_id UInt32,
    upvotes SimpleAggregateFunction(sum, UInt64),
    downvotes SimpleAggregateFunction(sum, UInt64),
    score SimpleAggregateFunction(sum, Int64)
)
ENGINE = AggregatingMergeTree()
ORDER BY post_id;
```

然后，创建一个包含 `SimpleAggregateFunction` 类型列的 materialized view：

```sql theme={null}
CREATE MATERIALIZED VIEW mv_vote_processor TO vote_aggregates
AS
SELECT
  post_id,
  -- sum 状态的初始值（upvote 时为 1，否则为 0）
  toUInt64(vote_type = 'upvote') AS upvotes,
  -- sum 状态的初始值（downvote 时为 1，否则为 0）
  toUInt64(vote_type = 'downvote') AS downvotes,
  -- sum 状态的初始值（upvote 时为 1，downvote 时为 -1）
  toInt64(vote_type) AS score
FROM raw_votes;
```

插入示例数据：

```sql theme={null}
INSERT INTO raw_votes VALUES
    (1, 'upvote'),
    (1, 'upvote'),
    (1, 'downvote'),
    (2, 'upvote'),
    (2, 'downvote'),
    (3, 'downvote');
```

使用 `SimpleState` 组合器查询该 materialized view：

```sql theme={null}
SELECT
  post_id,
  sum(upvotes) AS total_upvotes,
  sum(downvotes) AS total_downvotes,
  sum(score) AS total_score
FROM vote_aggregates -- 查询目标表
GROUP BY post_id
ORDER BY post_id ASC;
```

```response theme={null}
┌─post_id─┬─total_upvotes─┬─total_downvotes─┬─total_score─┐
│       1 │             2 │               1 │           1 │
│       2 │             1 │               1 │           0 │
│       3 │             0 │               1 │          -1 │
└─────────┴───────────────┴─────────────────┴─────────────┘
```

<div id="see-also">
  ## 另请参见
</div>

* [`sum`](/zh/reference/functions/aggregate-functions/sum)
* [`SimpleState 组合器`](/zh/reference/functions/aggregate-functions/combinators#-simplestate)
* [`SimpleAggregateFunction type`](/zh/reference/data-types/simpleaggregatefunction)