Oracle11g中ORA-01790

问题源于群里有人问如何让查询的结果值+1，方法其实很简单，直接在SQL语句中+1就可以，如果有空可以用NVL处理。

但是测试的时候我使用了UNION ALL（测试的字段是varchar2类型），结果报错ORA-01790。测试代码如下：

SELECT t.rearrangement13n+1 AS rearrangement13n  FROM t_plan t WHERE t.id = 97
UNION ALL
SELECT t.rearrangement13n FROM t_plan t WHERE t.id=97

　　之后感觉应该是进行类型转换，查了下文档。

OERR: ORA 1790 expression must have same datatype as corresponding expression
Error:  ORA 1790
Text:   expression must have same datatype as corresponding expression
-------------------------------------------------------------------------------
Cause:  A SELECT list item corresponds to a SELECT list item with a different
        datatype in another query of the same set expression.
Action: Check that all corresponding SELECT list items have the same datatypes.
        Use the TO_NUMBER, TO_CHAR, and TO_DATE functions to do explicit data
        conversions.

　　更改代码为：

SELECT to_char(t.rearrangement13n+1) FROM t_plan t WHERE t.id = 97
UNION ALL
SELECT t.rearrangement13n FROM t_plan t WHERE t.id=97

问题解决。但是Oracle是如何进行转换的呢？而且Oracle中如何输出select后的字段类型呢？在文档中没有找到。（待解决）