Filter a MySQL Result in Delphi

Question

I'm having an issue with a certain requirement to one of my Homework Assignments. I am required to take a list of students and print out all of the students with credit hours of 12 or more. The Credit hours are stored in a separate table, and referenced through a third table basically, a students table, a classes table with hours, and an enrolled table matching student id to Course id

I used a SUM aggregate grouped by First name from the tables and that all works great, but I don't quite understand how to filter out the people with less than 12 hours, since the SQL doesn't know how many hours each person is taking until it's done with the query.

my string looks like this

---

'SELECT Students.Fname, SUM(Classes.Crhrs) AS Credits 
FROM Students, Classes, Enrolled 
WHERE Students.ID = Enrolled.StudentID AND Classes.ID = Enrolled.CourseID 
GROUP BY Students.Fname;'

---

It works fine and shows the grid in the Delphi Project, but I don't know where to go from here to filter the results, since each query run deletes the previous.

Answer 1

由于这是一项家庭作业，因此我将给出一个简短的答案：查找有关HAVING的文档。

Answer 2

Beside getting the desired result directly from SQL as Martijn suggested, Delphi datasets have ways to filter data on the "client side" also. Check the Filter property and the OnFilter record.

Anyway, remember it is usually better to apply the best "filter" on the database side using the proper SQL, and then use client side "filters" only to allow for different views on an already obtained data set, without re-querying the same data, thus saving some database resources and bandwidth (as long as the data on the server didn't change meanwhile...)