Question

我有PostgreSQL数据库，我尝试打印所有用户（人）。

执行此查询时

-- show owners 
-- sorted by maximum cars amount
SELECT p.id
  FROM car c JOIN person p ON c.person_id = p.id
  GROUP BY p.id
  ORDER BY COUNT(p.name) ASC;

我按车辆数量排序所有车主

输出：3 2 4 1

当我尝试链接所有者ID时，所有订单都出错了。

SELECT *
FROM person p
WHERE p.id IN (
  SELECT p.id
  FROM car c JOIN person p ON c.person_id = p.id
  GROUP BY p.id
  ORDER BY COUNT(p.name) ASC);

输出：1 2 3 4和其他数据

你看到的比订单错了。所以我的问题是如何保存该命令？

Answer 1

取代subquery使用join。试试这个。

SELECT p.*
FROM   person p
       JOIN (SELECT p.id,
                    Count(p.NAME)cnt
             FROM   car c
                    JOIN person p
                      ON c.person_id = p.id
             GROUP  BY p.id) b
         ON p.id = b.id
ORDER  BY cnt ASC

Answer 2

解开这个烂摊子。首先聚合，稍后加入：

SELECT p.*
FROM   person p
JOIN  (
   SELECT person_id, count(*) AS ct
   FROM   car
   GROUP  BY person_id
   ) c ON c.person_id = p.id
ORDER  BY c.cnt;

无需加入person两次。如果您计算大多数或所有行，这应该是最快的对于小选择，相关子查询更快：

SELECT p.*
FROM   person p
ORDER  BY (SELECT count(*) FROM car c WHERE c.person_id = p.id)
WHERE  p.id BETWEEN 10 AND 20;  -- some very selective predicate

至于你的原文：IN右侧是集，忽略了元素的顺序，因此ORDER BY毫无意义subuery。

SQL：如何在sql查询中保存顺序？

2 个答案: