Counting daily distinct records

I have a table that looks like this in Google Big Query:

    custID  InteractionDate Purchased?  Sales
    1       20150312           F         0
    1       20150312           T         200
    1       20150314           T         150
    2       20150221           T         400
    2       20150221           F         0
    2       20150221           T         120

..that I need to transform to the following structure:

     custID InteractionDate Success Sales
     1     20150312          0.5    200
     1     20150314           1     150
     2     20150221         0.66    520

..where in this second table the success column is defined as

    count(purchase=Yes)/(count(purchase=Yes) + count(purchase=No)),

and sales is just the sum.

I'm quite new to BQ and no-SQL languages. I simply don't know how to do this, I've searched and seen examples that suggest using partition over but I simply haven't seen any example that does just this. Thanks a lot in advance.