Question

我想编写一个SQL语句，该语句根据CASE语句返回一个值，该语句有三种可能的结果。

1）如果列值以'C0'开头，则它返回表中两列的串联

2）如果列以'L0'开头，则取该行中另一列（_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。

我在下面写的声明工作正常，但第12行"Rollup_Project_Mapping"."Rollup_Project"除外，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。以下是错误消息：

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

当CASE语句的这部分为真时，有谁知道如何在其他表中返回值？

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

您只能使用一组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";

Answer 2

我根据@Gordon Linoff的答案修改了原始的SQL语句，以防有人想要它作为另一种选择：

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

当CASE语句为true时，SQL返回另一个表的值

2 个答案: