执行：

CREATE TABLE TMP_TBX_100_0_S3 AS SELECT t.* FROM (select t1.*,NULL AS sdate, NULL AS report_id from TMP_TBX_100_0_S4_1 t1 
union all 

select t2.* from TMP_TBX_100_0_S4_2 t2) t

报错：

ORA-00955: name is already used by an existing object

现象：

drop table TMP_TBX_100_0_S4 提示不错在表名或视图。

select * from TMP_TBX_100_0_S4

报错：

ORA-04063: view "DAILY.TMP_TBX_100_0_S4" 有错误
04063. 00000 -  "%s has errors"
*Cause:    Attempt to execute a stored procedure or use a view that has
           errors.  For stored procedures, the problem could be syntax errors
           or references to other, non-existent procedures.  For views,
           the problem could be a reference in the view's defining query to
           a non-existent table.
           Can also be a table which has references to non-existent or
           inaccessible types.
*Action:   Fix the errors and/or create referenced objects as necessary.

原因：

查了很多资料，百思不得其解，后来把

drop table TMP_TBX_100_0_S4 改成 drop view TMP_TBX_100_0_S4。成功了！

原来是，存在名为 TMP_TBX_100_0_S4 的视图，不存在名为TMP_TBX_100_0_S4 的表。

反之亦然