use sed to find matching text on left part of a line with " - " as the separator

: Need to search on left side of " - " in the line only
: file string setup to search criteria:
%groupbyname = "groupname - groupsongtitle"

The double quotes allow for a comma to be included in the search

sed -n "/%groupbyname/p" input_file > output_file

I'm using mks and/or Cgywin in Windows 10

Searching my cobwebs from running Xenix in the '80's!