I have two tables: students and results.

For students, the fields are `student_id, name, grade`  

For results, the fields are `student_id, score`

grade is like pre-school, one, two, three.... Like in a school.

As you see, `student_id` is in both tables.

I want a SQL query that returns average score per grade, like 

```
Pre-School => 50 
Kindergarten => 26 

```

etc.

I tried group by with left joins but nothing works.

The query I've been working on is as follows:

```
SELECT 
    cat.grade, 
    dog.score 
FROM (SELECT * FROM students GROUP BY grade) AS cat, results AS dog 
GROUP BY cat.grade

```

But this just gives me one result per grade, so `AVG()` does not give the right answer here.