How to Select a substring in Oracle SQL up to a specific character?

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

Result:

output
------
ABC

Use:

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

Reference:

• SUBSTR
• INSTR

Addendum

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