# 子查询字段

对于一般的子查询条件，例如`score > (SELECT avg(score) FROM t_user WHERE removed = ?)`，在OQM中被分为三个部分分别进行映射：

* score >
* SELECT avg(score) FROM t\_user
* WHERE clause

第一部分`score >`， 可以使用字段名`scoreGtXxx`来映射得到。\
谓词后缀之后定义的字符串仅用于区分重复的字段名，在映射时会被忽略。

第二部分包含一个列名和一个表名，这些属于不变的静态，DoytoQuery提供了两种注解来保存这些信息：

* `@Subquery`：定义子查询语句的列名和表名；
* `@SubqueryV2`：定义一个视图对象，用于配合字段赋值生成子查询语句。

第三部分是另一个WHERE子句，可以通过查询对象来映射。因此，我们将字段类型定义为对应的查询对象，并通过查询对象映射方法将字段的值映射为子查询中WHERE子句。

## 示例

**注解`@Subquery`：**

```java
@SuperBuilder
@NoArgsConstructor
public class UserQuery extends PageQuery {
    // ...
    
    @Subquery(select = "avg(score)", from = UserEntity.class)
    private UserQuery scoreLtAvg;

    @Subquery(select = "score", from = UserEntity.class)
    private UserQuery scoreLtAny;

    @Subquery(select = "score", from = UserEntity.class)
    private UserQuery scoreLtAll;

    @Subquery(select = "avg(score)", from = UserEntity.class)
    private UserQuery scoreGtAvg;
}
```

**注解`@SubqueryV2`：**

```java
@Getter
@Setter
@SuperBuilder
@NoArgsConstructor
@AllArgsConstructor
public class MinimumCostSupplierQuery extends PageQuery {
    private Integer p_size;
    private String p_typeEnd;
    private String r_name;
    @SubqueryV2(MinSupplyCostView.class)
    private SupplyCostQuery psSupplycost;

    @View(value = PartEntity.class, context = true)
    @View(PartsuppEntity.class)
    @View(SupplierEntity.class)
    @View(NationEntity.class)
    @View(RegionEntity.class)
    private static class MinSupplyCostView {
        @NoLabel
        private Integer minPs_supplycost;
    }
}
```


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