Teradata的字符串索引出界

Question

的这个查询的工作：

SELECT 
TOP 100  SUBSTRING(column_name FROM 6 FOR CHARACTER_LENGTH(column_name) - 5) AS X 
FROM  db_name.table_name 

但是，下面的查询（添加WHERE条款）不执行。

SELECT 
TOP 100  SUBSTRING(column_name FROM 6 FOR CHARACTER_LENGTH(column_name) - 5) AS X 
FROM  db_name.table_name 
WHERE  NOT EXISTS 
      (
      SELECT 1 
      FROM  db_name2.lookup_name H 
      WHERE  H.SRC_NUM1 = X 
         AND H.SRC_TYPE = 11 
      ) 

以上查询抛出

选择失败。 2663：SUBSTR：字符串标出TABLE_NAME.COLUMN_NAME

不过界，这下一个作品（原SELECT是嵌套）

SELECT  * 
FROM  (
      SELECT 
      TOP 100 SUBSTRING(column_name FROM 6 FOR CHARACTER_LENGTH(column_name) - 5) AS X 
      FROM  db_name.table_name 
      ) A 
WHERE  NOT EXISTS 
      (
      SELECT 1 
      FROM  db_name2.lookup_name H 
      WHERE  H.SRC_NUM1 = X 
         AND H.SRC_TYPE = 11 
      ) 

为什么会这样呢？我正在使用SQL助手来执行查询，但我怀疑它是相关的。

Answer 1

试图改变（也许错误时引起列名的lenght是小于6）：

SELECT 
TOP 100 CASE WHEN CHARACTER_LENGTH(column_name)>5 
      THEN SUBSTRING(column_name FROM 6 FOR CHARACTER_LENGTH(column_name) - 5) 
      ELSE NULL END AS X 
FROM  db_name.table_name