How to Extract a Substring Before an Underscore in Oracle SQL?

How to Extract a Substring Up to a Specific Character in Oracle SQL

Identifying the position of a character within a string can be a common challenge in data manipulation. To address this, the question explores a scenario where substrings up to a specific character, in this case the underscore ('_'), need to be extracted from a table column.

Position-Based Limitations of SUBSTRING

The initial approach of using the SUBSTRING function proves insufficient due to its reliance on position-based selection. As the underscore's position varies in the given examples, this method fails to consistently retrieve the desired substring.

The RTRIM Function: A Promising but Incomplete Solution

The consideration of the RTRIM function, intended to remove a specific set of characters from the right side, initially seems promising. However, it falls short in isolating the characters leading up to the underscore unless the RTRIM function is used in conjunction with another method.

A Combination of SUBSTR, INSTR, and NVL

The solution lies in combining the SUBSTR function with INSTR, which identifies the character's position, and NVL, which handles null values. The following code demonstrates this approach:

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

Understanding the Implementation

• The SUBSTR function extracts a substring based on the specified starting position (0) and length (INSTR('ABC_blah', '_')-1).
• The INSTR function finds the first occurrence of the underscore character and subtracts 1 to account for character indexing.
• The NVL function returns the substring if found (not null); otherwise, it returns the original string.

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