如何在 Oracle SQL 中选择特定字符的子字符串?

问题描述

假设我有一个表格列，其结果如下:

Say I have a table column that has results like:

ABC_blahblahblah
DEFGH_moreblahblahblah
IJKLMNOP_moremoremoremore

我希望能够编写一个查询，从所述表中选择这一列，但只返回下划线 (_) 字符之前的子字符串.例如:

I would like to be able to write a query that selects this column from said table, but only returns the substring up to the Underscore (_) character. For example:

ABC
DEFGH
IJKLMNOP

SUBSTRING 函数似乎不能胜任这项任务，因为它是基于位置的，并且下划线的位置各不相同.

The SUBSTRING function doesn't seem to be up to the task because it is position-based and the position of the underscore varies.

我想到了 TRIM 函数(特别是 RTRIM 函数):

I thought about the TRIM function (the RTRIM function specifically):

SELECT RTRIM('listofchars' FROM somecolumn) 
FROM sometable

但我不确定如何让它工作，因为它似乎只删除了某个列表/字符集，而我实际上只是在字符引导到下划线字符之后.

But I'm not sure how I'd get this to work since it only seems to remove a certain list/set of characters and I'm really only after the characters leading up to the Underscore character.

推荐答案

结合使用 SUBSTR、INSTR 和 NVL(对于没有下划线的字符串)将返回您想要的:

Using a combination of SUBSTR, INSTR, and NVL (for strings without an underscore) will return what you want:

SELECT NVL(SUBSTR('ABC_blah', 0, INSTR('ABC_blah', '_')-1), 'ABC_blah') AS output
  FROM DUAL

结果:

output
------
ABC

使用:

SELECT NVL(SUBSTR(t.column, 0, INSTR(t.column, '_')-1), t.column) AS output
  FROM YOUR_TABLE t

参考:

• SUBSTR
• INSTR

如果使用 Oracle10g+，您可以通过 REGEXP_SUBSTR 使用正则表达式.

If using Oracle10g+, you can use regex via REGEXP_SUBSTR.

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