Question

我可以对一行结果进行层次结构（使用来自previous question的查询），但是我不知道当它有多行时从哪里开始。

这是我的桌子：

public class NumGeneratorThreadSafe{

    List<Integer> numberPool = new ArrayList<>();
    private int counter = 0;

    public NumGeneratorThreadSafe() {
        for(int i = 1; i <= 1000 ; i++) {
            this.numberPool.add(i);
        }
        Collections.shuffle(this.numberPool);
    }

    public synchronized Integer getRandomNumber() {
        return this.numberPool.get(this.counter++);

    }

    public synchronized void resetCounter() {
        this.counter = 0;
    }

}

具有空


public class MultiThreadingRandom {

      public static void main(String[] args) throws InterruptedException {

         ExecutorService executorService = Executors.newFixedThreadPool(2);

         NumGeneratorThreadSafe numGenerator = new NumGeneratorThreadSafe();

         GeneRan r1 = new GeneRan(numGenerator);
         GeneRan r2 = new GeneRan(numGenerator);

         executorService.submit(r1);
         executorService.submit(r2);

         executorService.shutdown();

      }
}


class GeneRan  implements Runnable{

      private NumGeneratorThreadSafe numGenerator;

      public GeneRan(NumGeneratorThreadSafe numGenerator) {
         this.numGenerator = numGenerator;
      }

      @Override
      public void run() {
         int randInt = this.numGenerator.getRandomNumber();
         System.out.println(randInt);
      }
}

的

id| parent_id| name| -------|-----------|--------------------------| 1| [NULL]| United States| 2| [NULL]| Japan| 3| 1| New York| 4| 3| Brooklyn| 5| 6| Los Angeles| 6| 1| California| 7| 6| Dixon| 8| 2| Kyoto| 9| 2| Tokyo|是祖父母。那么，如何进行查询以生成这样的结果？

name

Answer 1

看起来像桌子上的一个简单的自我联接，因为您感兴趣的级别数量有限：

select co.name as country,
       p.name as "State/Province", 
       ct.name as city
from hierarchy co
  left join hierarchy p on p.parent_id = co.id
  left join hierarchy ct on ct.parent_id = p.id
where co.parent_id is null
order by co.name;

在线示例：https://rextester.com/UON36358

在Postgres中创建层次结构但有多个行是否可行？

1 个答案: