Can't say that there is a good definitive source....
In a nutshell, you are joining the tables together using columns that hold the same value. Say you are working with grades for students...you would have several tables say, student, grades, and classes.
say the columns were:
Student: Class: Grades:
StudentID, ClassID, GradeID
FName, ClassName, StudentID,
LName TeacherID ClassID,
Grade
The studentID is in all three tables, so you would link all the tables on StudentID, and you can link Class and Grades using the ClassID.
Now when you can build a report and group by the class or the student or the grade, or the teacher and all the values will be correct. You are linking the tables together on the like data.
HTH
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