Question

我有一个简单的group by语句，我需要检索具有为聚合计划的最大值的行的主键id。

select id, max(scheduled) from event group by customer_id, id;

这将返回所有结果，其中计划值为每个客户的最大（计划）。

如果我没有包含ID，则聚合按预期工作。

select max(scheduled) from event group by customer_id;

Answer 1

设计PK是独一无二的。因此，您可以获得与PK值一样多的组。你可以使用：

SELECT id, customer_id, scheduled AS max_scheduled
FROM event
WHERE (customer_id, scheduled) = (select customer_id, max(scheduled) 
                                  from event 
                                  group by customer_id);

Answer 2

实现目标的另一种方法是使用DISTINCT ON：

SELECT DISTINCT ON (customer_id) id, scheduled
FROM event
ORDER BY customer_id, scheduled DESC;

Postgres GROUP BY在包含主键时返回所有行

2 个答案: