When using bysort, difference between one or more variables in Stata : When I use the command bysort with just one variable and generate the mean of another variable, I get one set of values e.g. 42,43,39 etc.

Case 1.

    bysort date: egen dailymean = mean(temperature)// Gives mean temp for each day

When I use the command bysort with two variables and generate a similar mean, I get a different value e.g. 49,48,51 etc.  I want to understand what the values signifiy.

Case 2.

    bysort date isCentralPark: egen cpdailymean = mean(temperature)

In the first case, I think I am getting the mean of the temperature by the variable sorted by, date in other words daily mean temperature. 

In the second, am I getting the daily mean temperature in Central Park or something different?
