SQL ROUND AVG

Question

I have that 2 tables:

Evaluation
id_student  teste
----------- -----
1           16
1           10
1           20
1           13

Student
id          name
----------- ------
1           Jonh

And I want do the average of the column "teste" for the student with the id 1. I used that query:

select ROUND(AVG(e.teste),0) from Student s, Evaluation e
where s.id=e.id_student and s.id=1 group by s.nome

That query return the value 14, but if i go to the calculator and do (16+10+20+13) / 4 it gives me 14.75 I already tryed with the ROUND to round the number, the query should return 15 instaed of 14. Somebody know how I can soulve this? Thanks and sorry my english.

Answer 1

The problem is probably that the average is calculated using integer arithmetic. You don't specify your database, but some do use integer arithmetic. This has nothing to do with the round() .

Try this out:

select AVG(e.teste)
from Student s join
     Evaluation e
     on s.id = e.id_student 
where s.id = 1;

Notice the changes:

• Fixed the join use proper, explicit join syntax.
• Removed the round() .
• Removed the group by because you only seem to want to return one row.

This will return 14 and not 15 , because the calculation is integer 59 divided by integer 4 to return an integer -- so it is truncated not rounded. You can fix this by converting to some sort of decimal/float representation. Often the easiest way is just by multiplying by 1.0:

select AVG(e.teste * 1.0)
from Student s join
     Evaluation e
     on s.id = e.id_student 
where s.id = 1;

Once you have the average calculating correctly, you can apply round() if you like.