Question

I have three tables, with these fields:

classes: class_id | name | grade

classes_students: class_id | student_id

students: student_id | name

Classes has a n:m relationship with Students, so one class can have many students, and one student can listen to many classes. I want to select all students of a particular class, where class_id is 5.

Here are three queries that do the same thing. I run against MySQL latest version, InnoDB engine. Which one would show better performance and why?

Query A)

SELECT s.name
  FROM students s
  JOIN classes_students cs ON cs.student_id = s.student_id AND cs.class_id = 5

Query B)

SELECT s.name
  FROM students s
  JOIN classes_students cs ON cs.student_id = s.student_id
  JOIN classes c ON c.class_id = cs.class_id
 WHERE c.class_id = 5

Query C)

SELECT s.name 
 FROM students s 
 INNER JOIN classes_students cs ON cs.student_id = s.student_id
WHERE cs.class_id = 5

Answer 1

MySQL offers the EXPLAIN statement that will detail you the execution plan.

I did not run it but my bet is that the three queries are processed exactly the same way.

If you really need to tweak MySQL optimizer, you can have a look at the STRAIGHT_JOIN operator but MySQL is generally quite clever.