使用Ecto的相关子查询

Question

您将如何编写相关子查询Ecto？

例如,什么是SQL SELECT查询的等价物:

--Heaviest Lemmings per Group
SELECT ld.group_name, ld.id, ld.name, ld.weight
FROM lemming_data ld
WHERE weight = (
    SELECT max(weight)
    FROM lemming_data
    WHERE group = ld.group_name
);

在Ecto语法中？我不确定如何引用外部查询group_name.

Answer 1

在不使用片段的情况下,Ecto中无法进行与Co相关的子查询.所以它会是这样的:

from ld in "lemming_data",
  where: ld.weight in fragment("(SELECT max(weight) FROM lemming_data WHERE group = ?)", ld.group_name),
  select: ld # or {ld.group_name, ld.id, ld.name, ld.weight}

要么:

from ld in "lemming_data",
  where: fragment("? = (SELECT max(weight) FROM lemming_data WHERE group = ?)", ld.weight, ld.group_name),
  select: ld # or {ld.group_name, ld.id, ld.name, ld.weight}