Hi Barry Hunter! If the user wants to change the Department? How should it be? Change the name from all Employee?
Thanks very much for this discussion! Em 17/06/2011 16:46, "Barry Hunter" <[email protected]> escreveu: > > I think Ikai means, that you dont really need a 'Department' table. > > Just store "DepartmentName" directly as a string on your Employee model. > > > In a RDBS you traditionally 'normalize' - mainly to reduce database size. > > > In appengine and the datastore, you denormalize. Disk-space is 'cheap' > so duplicating the string in every record is not really an issue. > > http://en.wikipedia.org/wiki/Denormalization > > > Appengine is based on the premise of write once, read many, to making > your queries 'lightweight' is important too. Completely eliminating a > join makes the query much quicker to run. > > > > (unless you need to query on DepartmentName then you should make it as > not-indexed, otherwise it will eat up space in the indexes) > > > > On Fri, Jun 17, 2011 at 4:21 PM, Bruce Aloe <[email protected]> wrote: > > The reason i need to join these two kinds is because i need to know > > the department name that the employee works at as you can see the > > query "List all employees's Id (EmployeeId), name (Name), salary > > (Salary) > > and their department name (DepartmentName) for the ones earns more > > than 8000 US dollar". > > > > In SQL, i could do the query like this: > > select e.EmployeeId, e.Name, e.Salary, d.DepartmentName > > from Employee e, Department d > > where e.Salary > 8000 > > > > That is why i need to join these two kinds. > > > > Bruce > > > > On Jun 16, 1:26 pm, "Ikai Lan (Google)" <[email protected]> wrote: > >> It's not clear to me why you need a join here. If you're just doing it by > >> Department type, why not make the "Department" field a String? > >> > >> Ikai Lan > >> Developer Programs Engineer, Google App Engine > >> Blog:http://googleappengine.blogspot.com > >> Twitter:http://twitter.com/app_engine > >> Reddit:http://www.reddit.com/r/appengine > >> > >> > >> > >> On Thu, Jun 16, 2011 at 3:30 PM, Bruce Aloe <[email protected]> wrote: > >> > Hello, > >> > >> > In a simple case, suppose i have two kinds (tables) stored in GAE > >> > datastore: > >> > >> > Employee (EmployeeId, Name, Salary, StartDate, DepartmentId) > >> > Department(DepartmentId, DepartmentName) > >> > >> > For Employee kind, EmployeeId is the unique key and DepartmentId is > >> > the join attribute for Employee to make join with Department kind. For > >> > Department kind, DepartmentId is the unique key. Both Employee and > >> > Department kinds have quite a lot of tuples, let us say, more than 3 > >> > million tuples. > >> > >> > There is a need to make join query over both Employee and Department > >> > kinds in order to answer the query concerns the data from both kinds. > >> > For example a query could be "List all employees's Id, name, salary > >> > and their department name for the ones earns more than 8000 US > >> > dollar". > >> > >> > How can i make join query among different kinds to answer the query > >> > concerns different kinds? > >> > >> > Of course, there could be more than just two kinds stored in GAE > >> > datastore. If one wants to make join query among more than two kinds, > >> > how to handle that? > >> > >> > Thank you for your help! > >> > >> > Bruce > >> > >> > -- > >> > You received this message because you are subscribed to the Google Groups > >> > "Google App Engine" group. > >> > To post to this group, send email to [email protected]. > >> > To unsubscribe from this group, send email to > >> > [email protected]. > >> > For more options, visit this group at > >> >http://groups.google.com/group/google-appengine?hl=en.- Hide quoted text - > >> > >> - Show quoted text - > > > > -- > > You received this message because you are subscribed to the Google Groups "Google App Engine" group. > > To post to this group, send email to [email protected]. > > To unsubscribe from this group, send email to [email protected]. > > For more options, visit this group at http://groups.google.com/group/google-appengine?hl=en. > > > > > > -- > You received this message because you are subscribed to the Google Groups "Google App Engine" group. > To post to this group, send email to [email protected]. > To unsubscribe from this group, send email to [email protected]. > For more options, visit this group at http://groups.google.com/group/google-appengine?hl=en. > -- You received this message because you are subscribed to the Google Groups "Google App Engine" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/google-appengine?hl=en.
