> ## Documentation Index
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# minSimpleState

> minSimpleState 组合器的使用示例

<div id="description">
  ## 说明
</div>

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

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

让我们通过一个跟踪每日温度读数的表来看一个实际示例。对于每个位置，我们都希望保留已记录的最低温度。
使用带有 `min` 的 `SimpleAggregateFunction` 类型时，在遇到更低温度时会自动更新
存储的值。

创建用于存储原始温度读数的源表：

```sql theme={null}
CREATE TABLE raw_temperature_readings
(
    location_id UInt32,
    location_name String,
    temperature Int32,
    recorded_at DateTime DEFAULT now()
)
    ENGINE = MergeTree()
ORDER BY (location_id, recorded_at);
```

创建用于存储最低温度的聚合表：

```sql theme={null}
CREATE TABLE temperature_extremes
(
    location_id UInt32,
    location_name String,
    min_temp SimpleAggregateFunction(min, Int32),  -- 存储最低温度
    max_temp SimpleAggregateFunction(max, Int32)   -- 存储最高温度
)
ENGINE = AggregatingMergeTree()
ORDER BY location_id;
```

创建一个增量materialized view，使其作为已插入数据的插入触发器，
并维护每个位置的最低和最高温度值。

```sql theme={null}
CREATE MATERIALIZED VIEW temperature_extremes_mv
TO temperature_extremes
AS SELECT
    location_id,
    location_name,
    minSimpleState(temperature) AS min_temp,     -- 使用 SimpleState 组合器
    maxSimpleState(temperature) AS max_temp      -- 使用 SimpleState 组合器
FROM raw_temperature_readings
GROUP BY location_id, location_name;
```

插入一些初始温度读数：

```sql theme={null}
INSERT INTO raw_temperature_readings (location_id, location_name, temperature) VALUES
(1, 'North', 5),
(2, 'South', 15),
(3, 'West', 10),
(4, 'East', 8);
```

这些读数会由 materialized view 自动处理。我们来检查一下
当前状态：

```sql theme={null}
SELECT
    location_id,
    location_name,
    min_temp,     -- 直接访问 SimpleAggregateFunction 的值
    max_temp      -- 使用 SimpleAggregateFunction 无需调用终结函数
FROM temperature_extremes
ORDER BY location_id;
```

```response theme={null}
┌─location_id─┬─location_name─┬─min_temp─┬─max_temp─┐
│           1 │ North         │        5 │        5 │
│           2 │ South         │       15 │       15 │
│           3 │ West          │       10 │       10 │
│           4 │ East          │        8 │        8 │
└─────────────┴───────────────┴──────────┴──────────┘
```

再插入一些数据：

```sql theme={null}
INSERT INTO raw_temperature_readings (location_id, location_name, temperature) VALUES
    (1, 'North', 3),
    (2, 'South', 18),
    (3, 'West', 10),
    (1, 'North', 8),
    (4, 'East', 2);
```

查看新增数据后的最新极值：

```sql theme={null}
SELECT
    location_id,
    location_name,
    min_temp,  
    max_temp
FROM temperature_extremes
ORDER BY location_id;
```

```response theme={null}
┌─location_id─┬─location_name─┬─min_temp─┬─max_temp─┐
│           1 │ North         │        3 │        8 │
│           1 │ North         │        5 │        5 │
│           2 │ South         │       18 │       18 │
│           2 │ South         │       15 │       15 │
│           3 │ West          │       10 │       10 │
│           3 │ West          │       10 │       10 │
│           4 │ East          │        2 │        2 │
│           4 │ East          │        8 │        8 │
└─────────────┴───────────────┴──────────┴──────────┘
```

请注意，上面每个位置都有两个插入值。这是因为
parts 还没有合并 (并由 `AggregatingMergeTree` 聚合) 。要从这些部分状态中得到
最终结果，我们需要添加一个 `GROUP BY`：

```sql theme={null}
SELECT
    location_id,
    location_name,
    min(min_temp) AS min_temp,  -- 聚合所有 parts 的数据
    max(max_temp) AS max_temp   -- 聚合所有 parts 的数据
FROM temperature_extremes
GROUP BY location_id, location_name
ORDER BY location_id;
```

现在我们得到了预期的结果：

```response theme={null}
┌─location_id─┬─location_name─┬─min_temp─┬─max_temp─┐
│           1 │ North         │        3 │        8 │
│           2 │ South         │       15 │       18 │
│           3 │ West          │       10 │       10 │
│           4 │ East          │        2 │        8 │
└─────────────┴───────────────┴──────────┴──────────┘
```

<Note>
  使用 `SimpleState` 时，无需使用 `Merge` 组合器来合并
  部分聚合状态。
</Note>

<div id="see-also">
  ## 另请参阅
</div>

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