# 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，GoooQo提供了两种标签来保存这些信息： 一种是标签`subquery`，用于定义该子查询语句； 一种是标签`select`和`from`的组合，分别保存列名和表名。

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（v0.2.0+）

```go
ScoreLtAvg *UserQuery `subquery:"select avg(score) from User"`
ScoreLtAny *UserQuery `subquery:"SELECT score FROM User"`
ScoreLtAll *UserQuery `subquery:"select score from User"`
ScoreGtAvg *UserQuery `select:"avg(score)" from:"User"`
```


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