Question

Assume I have two tables,

   Student                           Test

Id    Name                   TestId   Type  StudentId
--    ----                   ------   ----  --------- 
1     Mark                     774    Hard     1  
2     Sam                      774    Hard     2 
3     John                     775    Easy     3

Now I have to find those students(the student id,name and testid) who have taken “hard” type of test in MySql.

Which one is better(In terms of performance)?.

1.Select student.id,student.name,test.testid 
  from student 
  join test 
  on test.studentid=student.id and test.type='hard'

2.Select student.id,student.name,test.testid 
  from student 
  join test 
  on test.studentid=student.id 
  where test.type='hard'

May I know the reason?(Assume there are millions of students and million types of tests)

Answer 1

Both are same but with different performance. I recommend to you use EXPLAIN PLAN for queries if you want to choose query with better performance.

Have look at Understanding the Query Execution Plan.

Also you can improve your queries with INDEXES for columns which you are using the most frequently in WHERE clause, you can have look at relational algebra also.