我尝试从三个不同的表中选择值。当我选择所有列时，它运行良好，但如果我选择特定列，SQL错误[42000]：JDBC-8027：列名不明确。请显示。
这是选择了所有运行良好的查询

SELECT 
*
FROM (SELECT x.*, B.*,C.* , COUNT(*) OVER (PARTITION BY x.POLICY_NO) policy_no_count 
FROM YIP.YOUTH_POLICY x
 LEFT JOIN
 YIP.YOUTH_POLICY_AREA B
 ON x.POLICY_NO = B.POLICY_NO
 LEFT JOIN
 YIP.YOUTH_SMALL_CATEGORY C
 ON B.SMALL_CATEGORY_SID = C.SMALL_CATEGORY_SID
ORDER BY x.POLICY_NO);

这是错误查询

SELECT DISTINCT
x.POLICY_NO,
x.POLICY_TITLE,
policy_no_count ,
B.SMALL_CATEGORY_SID,
C.SMALL_CATEGORY_TITLE 
FROM (SELECT x.*, B.*,C.* , COUNT(*) OVER (PARTITION BY x.POLICY_NO) policy_no_count 
        FROM YIP.YOUTH_POLICY x
        LEFT JOIN
         YIP.YOUTH_POLICY_AREA B
        ON x.POLICY_NO = B.POLICY_NO
        LEFT JOIN
         YIP.YOUTH_SMALL_CATEGORY C
        ON B.SMALL_CATEGORY_SID = C.SMALL_CATEGORY_SID
        ORDER BY x.POLICY_NO);

我正在尝试选择A.POLICY_NO值是否重复超过18行，是否希望将C.SMALL_CATEGORY_TITLE值更改为“ZZ”，以及是否希望将B.SMALL_CATEGORY_SID值转换为空。这就是为什么在如下查询中进行2次选择的原因

SELECT DISTINCT
x.POLICY_NO,
CASE WHEN (policy_no_count > 17) THEN 'ZZ' ELSE C.SMALL_CATEGORY_TITLE END AS C.SMALL_CATEGORY_TITLE,
CASE WHEN (policy_no_count > 17) THEN NULL ELSE B.SMALL_CATEGORY_SID END AS B.SMALL_CATEGORY_SID,
x.POLICY_TITLE
FROM (SELECT x.*, B.*,C.* , COUNT(*) OVER (PARTITION BY x.POLICY_NO) policy_no_count 
        FROM YIP.YOUTH_POLICY x
        LEFT JOIN
         YIP.YOUTH_POLICY_AREA B
        ON x.POLICY_NO = B.POLICY_NO
        LEFT JOIN
         YIP.YOUTH_SMALL_CATEGORY C
        ON B.SMALL_CATEGORY_SID = C.SMALL_CATEGORY_SID
        ORDER BY x.POLICY_NO);

如果我使用该查询，我得到SQL错误[42000]：JDBC-8006：缺少FROM关键字。¶第3行第80列的空错误。
我知道应该一步一步来解决，有没有办法选择具体栏目？