我不知道你为什么在里面使用整个查询 CAST ，但如果您使用 SELECT 以及 () 如下：（参见代码中的内联注解）

SELECT CAST (
            (SELECT CASE -- added ( and SELECT here
                WHEN D.DEGREE_HIERARCHY >= 40 
                    THEN 'Yes'

                WHEN D.DEGREE_HIERARCHY < 40 
                    THEN 'No'

                WHEN D.DEGREE_HIERARCHY IS NULL
                    THEN 'Unknown'
                ELSE
                    NULL
            END

        FROM DEGREE_CROSSWALK D, VPAA V
            WHERE
                V.FACULTY_DEGREE_CODE = D.DEGREE_CODE
                   AND V.PERSON_SKEY = SRC.PERSON_SKEY
                   AND V.CURRENT_DEGREE = '1') -- added ending bracket ) here

        AS VARCHAR2 (50)) FROM DUAL

我相信你需要这个：

select CASE WHEN D.DEGREE_HIERARCHY >= 40 THEN 
                 cast('Yes' as varchar(20))
            WHEN D.DEGREE_HIERARCHY < 40 THEN 
                 cast('No' as varchar(20))
            WHEN D.DEGREE_HIERARCHY IS NULL THEN 
                 cast('Unknown' as varchar(20))
            ELSE
                 NULL
       END name_of_column -- Here the case clause ends and you can name the column
FROM DEGREE_CROSSWALK D --add other tables and join condition here
WHERE --add other conditions of the where clause

另外，我建议您不要这样连接表：

FROM DEGREE_CROSSWALK D, VPAA V
WHERE V.FACULTY_DEGREE_CODE = D.DEGREE_CODE

但就像这样（只是一个例子）：

FROM DEGREE_CROSSWALK D
JOIN VPAA V ON V.FACULTY_DEGREE_CODE = D.DEGREE_CODE

首先将cast函数强制转换为数据类型，或者删除该强制转换。

SELECT CAST((SELECT CASE WHEN D.DEGREE_HIERARCHY >= 40 
                         THEN 'Yes'
                         WHEN D.DEGREE_HIERARCHY < 40 
                         THEN 'No'
                         WHEN D.DEGREE_HIERARCHY IS NULL
                         THEN 'Unknown'
                         ELSE NULL
                         END
             FROM DEGREE_CROSSWALK D, VPAA V
             WHERE V.FACULTY_DEGREE_CODE = D.DEGREE_CODE
             AND V.PERSON_SKEY = SRC.PERSON_SKEY
             AND V.CURRENT_DEGREE = '1') AS VARCHAR2 (50)) AS DOCTORATE
FROM DUAL