Extracting Substrings up to a Specific Character in Oracle SQL

In Oracle SQL, you may encounter situations where you need to extract a substring from a column up to a specific character. This can be achieved using various string functions provided by Oracle. In this tutorial, we will explore how to use these functions to extract substrings efficiently.

Understanding the Problem

Suppose you have a table with a column containing strings like ‘ABC_blahblahblah’, ‘DEFGH_moreblahblahblah’, and ‘IJKLMNOP_moremoremoremore’. You want to write a query that selects this column but only returns the substring up to the underscore (_) character. For example, for the given strings, you would expect ‘ABC’, ‘DEFGH’, and ‘IJKLMNOP’ as outputs.

Using SUBSTR and INSTR Functions

One approach is to use a combination of the SUBSTR and INSTR functions. The INSTR function returns the position of the first occurrence of a specified character in a string, while the SUBSTR function extracts a portion of a string.

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

This query works as follows:

• The INSTR function finds the position of the underscore (_) in the string ‘ABC_blah’.
• The SUBSTR function then extracts a substring from the beginning of the string up to but not including the character at the position found by INSTR.

However, if the input string does not contain an underscore, INSTR returns 0, and SUBSTR will return NULL. To handle this scenario, you can use the NVL function:

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

Using REGEXP_SUBSTR Function

Another approach is to use the REGEXP_SUBSTR function, which allows you to extract substrings based on regular expressions. This function is particularly useful for extracting patterns from strings.

SELECT REGEXP_SUBSTR('ABC_blah', '[^_]+', 1, 1) AS output
FROM DUAL;

In this query:

• The pattern [^_]+ matches any character that is not an underscore one or more times.
• The third argument to REGEXP_SUBSTR, which is 1, specifies the starting position of the search in the string (starting from 1).
• The fourth argument, also 1, indicates that we want the first occurrence of the pattern.

This method is concise and can handle cases where the input string does not contain an underscore without needing additional functions like NVL.

Applying to Table Columns

To apply these solutions to a table column, you replace the hardcoded strings with your column name. For example:

SELECT NVL(SUBSTR(your_column, 1, INSTR(your_column, '_') - 1), your_column) AS output
FROM your_table;

Or using REGEXP_SUBSTR:

SELECT REGEXP_SUBSTR(your_column, '[^_]+', 1, 1) AS output
FROM your_table;

Conclusion

Extracting substrings up to a specific character in Oracle SQL can be efficiently achieved using the combination of SUBSTR and INSTR functions or the REGEXP_SUBSTR function. The choice between these methods depends on your specific requirements and preferences, including handling cases where the delimiter is not present in the string.