How to use MySQL triggers this way?
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I have two tables in my database. The first is quite simple, but the second table is (a bit) complicated. 

My data is stored this way (in the second table): 1_2_3 (data separated with _).

I need a trigger that updates this string in the second table (when a specific value is updated in the first table), but only a specific substring, so.. if I want to change only the first substring ('1' in the current example), the 2 remaining substrings should not change, nor reset to their default value.

I looked for mysql split methods, or substring methods, but have not found anything.