使用 "AS" 字段的条件查询

Question

SELECT "public"."mv_tags_per_org"."count" AS "count", "public"."mv_tags_per_org"."tag_name" AS "Tag Name",
CASE 
    WHEN "public"."mv_tags_per_org"."ngo_id" = 30 then 'SSS'
    WHEN "public"."mv_tags_per_org"."ngo_id" = 33 then 'PF'
    WHEN "public"."mv_tags_per_org"."ngo_id" = 34 then 'DS'
    ELSE 'Maybe'
END AS "NPO"
FROM "public"."mv_tags_per_org"

WHERE "NPO???" = "SSS"

上面你可以看到我的代码。当您删除 "WHERE" 函数时，它当前准确地返回我想要的输出。我正在添加 "WHERE" 函数并尝试访问我创建的名为 "NPO" 的新列。 SQL 编辑器似乎不存在该列，但当查询为运行时它确实存在。我如何访问它？

谢谢！

Answer 1

将您的查询包含在 "table expression" 中，这样您就可以生成命名列。然后就可以在WHERE子句中使用了：

select *
from ( -- table expression 'x' starts here
  SELECT
    "public"."mv_tags_per_org"."count" AS "count",
    "public"."mv_tags_per_org"."tag_name" AS "Tag Name",
  CASE 
    WHEN "public"."mv_tags_per_org"."ngo_id" = 30 then 'SSS'
    WHEN "public"."mv_tags_per_org"."ngo_id" = 33 then 'PF'
    WHEN "public"."mv_tags_per_org"."ngo_id" = 34 then 'DS'
    ELSE 'Maybe'
  END AS "NPO"
  FROM "public"."mv_tags_per_org"
) x
WHERE "NPO" = 'SSS'

注意："table expressions"也被不同的团队称为"derived tables"和"inline views"。

Answer 2

WHERE 子句不能与 SELECT 子句中定义的列别名相关（因为前者在之前后者被评估）。

这对您的用例来说并不重要，可以简化为：

SELECT 
    "public"."mv_tags_per_org"."count" AS "count",    
    "public"."mv_tags_per_org"."tag_name" AS "Tag Name",
CASE 
    WHEN "public"."mv_tags_per_org"."ngo_id" = 30 then 'SSS'
    WHEN "public"."mv_tags_per_org"."ngo_id" = 33 then 'PF'
    WHEN "public"."mv_tags_per_org"."ngo_id" = 34 then 'DS'
    ELSE 'Maybe'
END AS "NPO"
FROM "public"."mv_tags_per_org"
WHERE "public"."mv_tags_per_org"."ngo_id" = 30

使用 "AS" 字段的条件查询

Conditional Query Using "AS" field

sql

select

where-clause

column-alias