# 列映射

视图对象的字段不仅用于映射不同表的列，还用于表示聚合表达式和分组子句。

## 聚合列映射

聚合函数的应用是聚合查询的核心功能。所有通过聚合列查询的数据都需由视图类中定义的字段保存，因此我们需要将字段名映射为聚合列，使用前缀映射是一种很好的选择。

| 前缀         | 聚合函数名        | 字段名             | 聚合列表达式                                 |
| ---------- | ------------ | --------------- | -------------------------------------- |
| sum        | sum          | sumScore        | sum(score) AS sumScore                 |
| max        | max          | maxScore        | max(score) AS maxScore                 |
| min        | min          | minScore        | min(score) AS minScore                 |
| avg        | avg          | avgScore        | avg(score) AS avgScore                 |
| first      | first        | firstScore      | first(score) AS firstScore             |
| last       | last         | lastScore       | last(score) AS lastScore               |
| stdDevPop  | stddev\_pop  | stdDevPopScore  | stddev\_pop(score) AS stdDevPopScore   |
| stdDevSamp | stddev\_samp | stdDevSampScore | stddev\_samp(score) AS stdDevSampScore |
| stdDev     | stddev       | stdDev          | stddev(score) AS stdDev                |
| addToSet   | addToSet     | addToSetScore   | addToSet(score) AS addToSetScore       |
| push       | push         | pushScore       | push(score) AS pushScore               |
| count      | count        | countScore      | count(score) AS countScore             |
| count      | count        | count           | count(\*) AS count                     |

**示例** 如果要计算名为 `score` 的列的平均值，使用 `avg` 函数，定义字段名为 `avgScore`，遵循聚合前缀+列名的命名规则，最终映射为 `avg(score) AS avgScore`。

## 聚合表达式映射

对于无法直接用字段名表示的聚合表达式，可以通过注解指定具体的表达式，将表达式映射为列名，字段名作为标签。

**示例** 定义字段时添加表达式注解：

```java
@Column(name = "sum(l_extendedprice*(1-l_discount))")
private BigDecimal sum_disc_price;
```

该字段映射为：

```sql
sum(l_extendedprice*(1-l_discount)) AS sum_disc_price
```

**完整示例** 请参见表格：

| LN | 对象代码                                                         | SQL 语句                                                 |
| -- | ------------------------------------------------------------ | ------------------------------------------------------ |
| 1  | @View(CustomerEntity.class)                                  | SELECT                                                 |
| 2  | @View(OrdersEntity.class)                                    |                                                        |
| 3  | @View(LineitemEntity.class)                                  |                                                        |
| 4  | public class ShippingPriorityView {                          |                                                        |
| 5  | @GroupBy                                                     |                                                        |
| 6  | private String l\_orderkey;                                  | l\_orderkey,                                           |
| 7  | @Column(name = "SUM(l\_extendedprice \* (1 - l\_discount))") | SUM(l\_extendedprice \* (1 - l\_discount)) AS revenue, |
| 8  | private Double revenue;                                      |                                                        |
| 9  | @GroupBy                                                     |                                                        |
| 10 | private Date o\_orderdate;                                   | o\_orderdate,                                          |
| 11 | @GroupBy                                                     |                                                        |
| 12 | private String o\_shippriority;                              | o\_shippriority                                        |
| 13 | }                                                            | FROM customer, orders, lineitem                        |
|    | // @View(CustomerEntity.class)                               | WHERE o\_custkey = c\_custkey                          |
|    | // @View(OrdersEntity.class)                                 | AND l\_orderkey = o\_orderkey                          |
|    | // @View(LineitemEntity.class)                               |                                                        |
| 1  | public class ShippingPriorityQuery extends PageQuery {       |                                                        |
| 2  | private String c\_mktsegment;                                | AND c\_mktsegment = ?                                  |
| 3  | private Date o\_orderdateLt;                                 | AND o\_orderdate < ?                                   |
| 4  | private Date l\_shipdateGt;                                  | AND l\_shipdate > ?                                    |
| 5  | }                                                            | GROUP BY l\_orderkey, o\_orderdate, o\_shippriority    |
|    | // PageQuery.sort = "revenue,DESC;o\_orderdate"              | ORDER BY revenue DESC, o\_orderdate                    |


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://query.docs.doyto.win/zh/object-concepts/view-object/column-mapping.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.