QueryDSL JPA- 与 group by 无关的自连接

Question

有什么方法可以使用 QueryDSL 获取此查询吗？

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来进行自连接：

// 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，您只需使用元组 class：

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()));
}

或者您可以使用 ConstructorExpression 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;
    }
}

QueryDSL JPA- 与 group by 无关的自连接

QueryDSL JPA- Self join without relationship with group by

java

jpa

jpql

querydsl