一个关于子查询的问题求解决！

Question

SELECT CASE
          WHEN T1.A = T2.A THEN
           (SELECT T3.A FROM (SELECT 1 AS A FROM DUAL) T3 WHERE T2.A = T3.A)
          ELSE
           0
        END AS TEST
   FROM (SELECT 1 AS A FROM DUAL) T1, (SELECT 1 AS A FROM DUAL) T2
  WHERE EXISTS (SELECT T4.A
                  FROM (SELECT T3.A
                          FROM (SELECT 1 AS A FROM DUAL) T3
                         WHERE T2.A = T3.A) T4)

上面的SQL在执行时，提示T2.A是无效的标识符，如果我把Exist里面套的那个Select T4.A From（）T4这层去掉，就可以执行，
求解释！！！！

Answer 1

我这里报的不是这个错，你这是要查询什么？
SELECT 1 AS A FROM DUAL 这句语句写的有问题