Answer by totymedli for Split a string at a specific character in SQL

tl;dr

Use split_part which was purposely built for this:

split_part(string, '_', 1)

Explanation

Quoting this PostgreSQL API docs:

SPLIT_PART() function splits a string on a specified delimiter and returns the nth substring.

The 3 parameters are the string to be split, the delimiter, and the part/substring number (starting from 1) to be returned.

So if you have a field named string that contains stuff like AB_XXX and you would like to get everything before _, then you split by that and get the first part/substring: split_part(string, '_', 1).