SQL 在 CASE 语句为真时返回另一个表的值

Question

我想编写一个 SQL 语句，它基于具有三种可能结果的 CASE 语句返回一个值。

1) 如果列值以 'C0' 开头，则返回表中两列的串联

2) 如果该列以“L0”开头，则取该行另一列 (_PARENT_PROJECT_NUMBER) 中的值，并查看该值是否位于另一个表中。如果不是，则返回值 _PARENT_PROJECT_NUMBER。

3)如果 _PARENT_PROJECT_NUMBER 确实驻留在另一个表中，则返回另一个表“Rollup_Project_Mapping”中“Rollup_Project”列中的值。

除了第 12 行 "Rollup_Project_Mapping"."Rollup_Project"，我在下面写的语句工作正常，如果在另一个表中找到 _PARENT_PROJECT_NUMBER，我希望它返回另一个表的值.这是错误消息:

ERROR:  syntax error at or near "FROM"
LINE 12:      FROM "Rollup_Project_Mapping"."Rollup_Project"

有谁知道当这部分 CASE 语句为真时我如何返回另一个表中的值？

SELECT
CASE 
  WHEN LEFT("_PROJECT_NUMBER",2)='C0' THEN
    '('||"_PROJECT_NUMBER"||') '||"_PROJECT_DESCRIPTION"
  WHEN LEFT("_PROJECT_NUMBER",2)='L0' THEN
    CASE
      WHEN (SELECT "Rollup_Project" FROM "Project_Rollup_Mapping" WHERE "PA_ACCOUNTED_TEST"."_PARENT_PROJECT_NUMBER" = "Project_Rollup_Mapping"."Project") IS NOT NULL THEN
        "Rollup_Project_Mapping"."Rollup_Project"
    ELSE "_PARENT_PROJECT_NUMBER"
    END
END AS Rollup_Project2
FROM "PA_ACCOUNTED_TEST";

Answer 1

您可以只使用一组 WHEN/THEN 语句和 EXISTS:

SELECT (CASE WHEN LEFT("_PROJECT_NUMBER",2) = 'C0'
             THEN '('||"_PROJECT_NUMBER"||') '||"_PROJECT_DESCRIPTION"
             WHEN LEFT("_PROJECT_NUMBER",2) = 'L0' AND
                  EXISTS (SELECT "Rollup_Project" FROM "Project_Rollup_Mapping" WHERE "PA_ACCOUNTED_TEST"."_PARENT_PROJECT_NUMBER" = "Project_Rollup_Mapping"."Project")
                  THEN (SELECT "Rollup_Project" FROM "Project_Rollup_Mapping" WHERE "PA_ACCOUNTED_TEST"."_PARENT_PROJECT_NUMBER" = "Project_Rollup_Mapping"."Project")
             ELSE "_PARENT_PROJECT_NUMBER"
       END) AS Rollup_Project2
FROM "PA_ACCOUNTED_TEST";