Your column name “column_name” is of type text. The number 255 is an integer. Since those datatypes don’t match the coalesce function fails. If the data in “column_name” can always be cast to an integer then you can use
Select coalesce(column_name::integer, 255);
Or you can cast 255 to a text string of ‘255’ with this:
Select coalesce(column_name, '255'::text);
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