在Oracle中加入子句和Where子句

Question

Assume I have two tables,

   Student                           Test

Id    Name                   TestId   Score  StudentId
--    ----                   ------   ----  ---------  
1    Mark                      774     100       1   
2     Sam                      774      89       2  
3    John                      775      78       3

Now I have to print student name,test id and score of each student.

I know both of them produce the same results.But which one is better in terms of performance?Does the second one find cartesian product and then apply filter(where clause)?

1.Select test.testid,student.name,test.score 
  from student 
  join test 
  on test.studentid=student.id

2.Select test.testid,student.name,test.score  
  from student,test 
  where test.studentid=student.id

Answer 1

I'm not going to answer your question directly but rather provide you with a tool to solve such problems in the future.

Oracle offers ways to see how queries are executed. You can see how the access to your tables is performed, how much time a query execution takes, whether indexes are used or not, etc.

The commands are:

EXPLAIN PLAN and SET AUTOTRACE .

In your case, it would be as simple as this:

EXPLAIN PLAN FOR
Select test.testid,student.name,test.score 
from student 
join test 
on test.studentid=student.id;

EXPLAIN PLAN FOR
Select test.testid,student.name,test.score  
from student,test 
where test.studentid=student.id;

Or using autotrace:

Set autotrace on;

Select test.testid,student.name,test.score 
from student 
join test 
on test.studentid=student.id;

Select test.testid,student.name,test.score  
from student,test 
where test.studentid=student.id;

In case of EXPLAIN PLAN , the results are kept in a special table that you can query to see them. Check the documentation I linked to in order to see what can be done with it.

Answer 2

Both queries will have equal execution plans (at least in MS SQL they do). Second query will be optimized