A SQL lookup on two tables;

Question

The database is here: http://sqlfiddle.com/#!9/bf0171

I want to find all students in "Prof.David" class. However, the lookup result is:

Select Student.name from Student, Teacher where Teacher.s_id =Student.id and Teacher.name="Prof. David";
+------+
| name |
+------+
| Tom  |
| John |
| Mary |
| Tom  |
| John |
| Mary |
+------+

I suppose the result should be "Tom" and "John" only. What's the problem?

Answer 1

Without a join criteria between Student and Teacher you'll get a Cartesian product (all records from Student combined with all records from Teacher ).

You probably meant:

SELECT
    Student.name
FROM Student s
JOIN Teacher t
  ON t.s_id = s.id
WHERE Teacher.name="Prof. David";

For new learners of SQL, I would strongly recommend explicitly using using JOIN , LEFT JOIN , etc instead of implicitly joining in the WHERE clause. It will help reduce the number of accidental Cartesian products you execute.

Answer 2

The issue is that you have two tables and you are performing a cartesian join on those tables with your query. Therefore, you are getting 3x2 = 6 rows in your results, where for each teacher you are showing the names of all 3 students. You must join your tables in a logical way based on the foreign key relationships in your schema.

For example:

Select A.field1, B.field2 from A join B on A.id = B.a_id

Answer 3

To see what the problem is, try this:

Select Student.name, Teacher.name from Student, Teacher

You will get a result where every student is combined with every teacher, whether one has anything to do with the other or not. You need to add a condition that checks if the teacher and student are related:

Select Student.name, Teacher.name from Student, Teacher where Teacher.s_id = Student.id

Once you have that you can add more conditions, like listing only the students of a given teacher.

Select Student.name from Student, Teacher where Teacher.s_id = Student.id and Teacher.name=...

Answer 4

You need to add a join clause to the where. I suppose you are trying to select students who have a particular teacher in a particular class. So, for example if you had:

table Student with columns: student_id, name
table Teacher with columns: teacher_id, name
table Class with columns: class_id, class_name, teacher_id
table Class_Assignment with columns class_id, student_id

Then your query would be:

Select Student.name from Student, teacher, class_assignment
   where Student.student_id = class_assignment.student_id
   and Teacher.teacher_id = class_assignment.teacher_id
   and Teacher.name="Prof. David";