如何计算postgres中的多个连接行？

Question

我有4张桌子：A，B，C，D。

• 表B具有A
的外键
• 表C具有A
的外键
• 表D具有C的外键

到目前为止我可以查询：

• 我知道如何从A中选择所有行并计算B的连接行数。
• 我知道如何从A中选择所有行并从D（通过C）计算连接行的数量。

现在我想在一个select中合并这两个查询，并在那里显示count_b和count_d。但结果选择显示错误计数。以下是我的表现方式：

SELECT
  "A"."id",
  COUNT("B"."id") AS "B_count",
  COUNT("D"."id") AS "D_count"
FROM "users" AS "A" LEFT OUTER JOIN "B" AS "B"
    ON "A"."id" = "B"."a__id"
  LEFT OUTER JOIN "C" AS "C"
    ON "A"."id" = "C"."a_id"
  LEFT OUTER JOIN "D" AS "D"
    ON "C"."id" = "D"."c_id"
GROUP BY "A"."id"

GROUP BY可能存在问题。我似乎需要为每个计数单独编写组。我正在寻找没有嵌套选择的结果（因为我仍然需要将它映射到ORM，不要问为什么）。是否可以使用distinct by进行存档？

Answer 1

由于您要加入3个表，因此您应该在COUNT中使用DISTINCT：

SELECT
  "A"."id",
  COUNT(DISTINCT "B"."id") AS "B_count",
  COUNT(DISTINCT "D"."id") AS "D_count"
FROM "users" AS "A" LEFT OUTER JOIN "B" AS "B"
    ON "A"."id" = "B"."a__id"
  LEFT OUTER JOIN "C" AS "C"
    ON "A"."id" = "C"."a_id"
  LEFT OUTER JOIN "D" AS "D"
    ON "C"."id" = "D"."c_id"
GROUP BY "A"."id"