横向查询语法

Question

我正在尝试横向处理 Postgres 9.5.3 查询。

select  b_ci."IdOwner",
ci."MinimumPlaces",
ci."MaximumPlaces",
(select count(*) from "LNK_Stu_CI" lnk
where lnk."FK_CourseInstanceId" = b_ci."Id") as "EnrolledStudents",

from "Course" c
join "DBObjectBases" b_c on c."Id" = b_c."Id"

join "DBObjectBases" b_ci on b_ci."IdOwner" = b_c."Id"
join "CourseInstance" ci on ci."Id" = b_ci."Id",

lateral (select ci."MaximumPlaces" - "EnrolledStudents") x

我希望最右边的列是该行的“MaximumPlaces”-“EnrolledStudents”的结果，但我正在努力让它发挥作用。目前 PG 正在抱怨“EnrolledStudents”不存在——这正是“横向”的意义所在，不是吗？

select  b_ci."IdOwner",
ci."MinimumPlaces",
ci."MaximumPlaces",
(select count(*) from "LNK_Stu_CI" lnk
where lnk."FK_CourseInstanceId" = b_ci."Id") as "EnrolledStudents",
lateral (select "MaximumPlaces" - "EnrolledStudents") as "x"

from "Course" c
join "DBObjectBases" b_c on c."Id" = b_c."Id"

join "DBObjectBases" b_ci on b_ci."IdOwner" = b_c."Id"
join "CourseInstance" ci on ci."Id" = b_ci."Id"

如果我尝试在 select 中内联横向子句（如上所示），它也会感到不安并给我一个语法错误 - 那么它去哪里了？

谢谢，

亚当。

Answer 1

您错过了 LATERAL 的重点。它可以访问 FROM 子句中的表中的列，但不能访问 SELECT 子句中定义的别名。

如果您想访问 SELECT 子句中定义的别名，您需要添加另一个查询级别，或者使用 FROM 子句中的子查询（AKA 派生表）或使用 CTE (Common Table Expression)。由于 PostgreSQL 中的 CTE 充当优化围栏，因此我强烈建议在这种情况下使用子查询，例如：

select
    -- get all columns on the inner query
    t.*,
    -- get your new expression based on the ones defined in the inner query
    t."MaximumPlaces" - t."EnrolledStudents" AS new_alias
from (
    select  b_ci."IdOwner",
    ci."MinimumPlaces",
    ci."MaximumPlaces",
    (select count(*) from "LNK_Stu_CI" lnk
    where lnk."FK_CourseInstanceId" = b_ci."Id") as "EnrolledStudents",

    from "Course" c
    join "DBObjectBases" b_c on c."Id" = b_c."Id"

    join "DBObjectBases" b_ci on b_ci."IdOwner" = b_c."Id"
    join "CourseInstance" ci on ci."Id" = b_ci."Id"
) t