QueryDSL JPA-与group by无关的自加入

Question

有什么办法可以用 QueryDSL 获取这个查询吗？

select
  person.name,
  count(neighbors.*)
from person as neighbors
where person.address = neighbor.address
group by person.name

其中address不是 FK。

Answer 1

第一个问题的关键是使用别名能够进行自连接：

// static instance uses default alias "person"
QPerson person = QPerson.person;
QPerson neighbor = new QPerson("neighbor");

JPAQuery query = new JPAQuery(em)
    .from(person, neighbor)
    .where(person.adress.eq(neighbor.adress))
    .groupBy(person.name);

要获得结果，您可以简单地使用 Tuple 类：

List<Tuple> results = query.list(person.name, neighbor.count());
for (Tuple row : result) {
    System.out.println("name: " + row.get(person.name));
    System.out.println("count: " + row.get(neighbor.count()));
}

或者你可以使用 aConstructorExpression来做这样的事情：

List<MyResult> results = query.list(ConstructorExpression.create(
        MyResult.class, person.name, neighbor.count()));

public class MyResult {
    private String name;
    private long count;
    public MyResult(String name, long count) {
        this.name = name;
        this.count = count;
    }
}