如何使用Google App Engine中的条件解决聚合查询

Suppose, In my app, I ask users to input some string. A user can input string multiple times. Whenever any user inputs a string, I log it in the database along with the day. Many strings can be same, even though inputted by different users. In the home page, I need to give the interface such that any user can query for top n (say 50) strings in any time period (say between last 45 days, or 10 Jan 2012 to 30 Jan 2012). If it was SQL, I could have written query like:Suppose, In my app, I ask users to input some s