Sql 当连接两个表中的聚合时，如何避免笛卡尔积？

我需要连接两个表中的聚合，但得到的是笛卡尔积。我怎样才能避免呢

SELECT TO_CHAR(c.date, 'YYYY-MM') AS "Year-Month",
   SUM(c.amount) as c_amount,
   SUM(o.amount) as o_amount
FROM c JOIN o ON c.common = o.common AND c.date = o.date
GROUP BY TO_CHAR(c.date, 'YYYY-MM')
ORDER BY TO_CHAR(c.date, 'YYYY-MM');

预骨料：

select c.yyyymm, c.c_amount, o.o_amount
from (select to_char(c.date, 'YYYY-MM') as yyyymm,
             sum(c.amount) as c_amount
      from c
      group by to_char(c.date, 'YYYY-MM')
     ) c left join
     (select to_char(c.date, 'YYYY-MM') as yyyymm,
             sum(o.amount) as o_amount
      from c join
           o
           on c.common = o.common
      group by to_char(c.date, 'YYYY-MM')
     ) o
     on c.yyyymm = o.yyyymm;

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请提供样本数据和期望的结果。这很有效！非常感谢。你会推荐什么资源来学习这些技巧？@oavaldezi。这相当棘手。您可能只需要使用数据方面的实践和经验。