The comma you see is the thousands-separator that gets put in place because of your locale setting.
Note: it's only the way the data is represented - it's not IN the data.
This happens because the output data type for your YEAR is apparently set to INTEGER - you want to check that.
Anyhow, the important bit here is: this data representation is just what the standard formatting of the data preview does (see also Community criss cross puzzle posting in multiple locales for a more complete discussion on that).
As this cannot be the final output for your query anyhow, you can safely ignore this thousands separator.