WHERE子句和ON子句中的条件

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.

Answer 2

Those queries are exactly the same and should have exactly the same query plan. It is unlikely that there will be any measurable difference in execution time.

There will however be a difference if you change JOIN to LEFT JOIN . In that case you need to use the first version. The second version will not give the same results.

Answer 3

Performance-wise these two are about the same. You can verify that by using the EXPLAIN command.

So the question becomes one of clarity of expression of your data model. It may seem like angels dancing on the head of a pin. But, if you're pulling all the test results with your JOIN commmand, and then asking for a subset, go with WHERE type='hard' .

But if, conceptually, you're pulling only the hard ones, then go with the ON formulation.

Notice that these two will work differently if you use LEFT JOIN . You'll get a row for each student that hasn't ever taken a hard test, as well as a row for each hard test each student has taken if you use the ON formulation with LEFT JOIN .