The Federal Statistical Office of Switzerland sends compulsory salary surveys to the enterprises itself. The collected data is then published in the Salarium [1]. It is based on more than 750'000 salaries from the private sector and runs since 1994. You can read more about the methodology in the PDF [2]

I worked as a Software Developer for a company that used the data as base for their salary negotiations and it worked quite well.

[1] https://www.bfs.admin.ch/bfs/en/home/statistics/work-income/...

[2] https://www.bfs.admin.ch/bfsstatic/dam/assets/6937/master