当 CASE 语句为 true 时返回另一个表的值的 SQL

Question

我想编写一条 SQL 语句，该语句根据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，则它返回表中两列的串联

2) 如果该列以“L0”开头，则获取该行中另一列 (_PARENT_PROJECT_NUMBER) 中的值，并查看该值是否驻留在另一个表中。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。

我在下面编写的语句工作正常，但第 12 行除外，"Rollup_Project_Mapping"."Rollup_Project"在该行中，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 CASE 语句的这一部分为真时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

您可以仅使用一组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";