return a text agreggate from a subselect

Hi,

I am trying to convert a subselect that has several matches into one
string comprising all matches concatenated:

The subselect: (SELECT company_name FROM table) 

The output I'd like: 'Company1|Company2|Company3|etc.'

Should I write a function or a new agreggate for this or is there is
simpler way?

Thanks in advance,

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