如何计算postgres中的多个联接行？

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的内容：

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

现在，我想将这两个查询合并为一个选择，并在其中显示count_b和count_d。但是结果选择显示错误的计数。这是我的方法：

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"

可能存在问题GROUP BY。看来我需要为每个计数编写单独的分组依据。我正在寻找没有嵌套选择的结果（因为我仍然需要将其映射到ORM，所以不要问为什么）。是否可以存档distinct by？

Answer 1

由于您要加入3个表，因此应在COUNT内使用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"