Where and group conflict? - mysql

Question

I have a table like this:
mysql> select * from studentscore;
+------------+-----------+-------+
| student_id | cource_id | score |
+------------+-----------+-------+
| 1 | 1 | 80 |
| 1 | 2 | 90 |
| 1 | 3 | 85 |
| 1 | 4 | 78 |
| 2 | 2 | 53 |
| 2 | 3 | 77 |
| 2 | 5 | 80 |
| 3 | 1 | 71 |
| 3 | 2 | 70 |
| 3 | 4 | 80 |
| 3 | 5 | 65 |
| 3 | 6 | 75 |
| 4 | 2 | 90 |
| 4 | 3 | 80 |
| 4 | 4 | 70 |
| 4 | 6 | 95 |
| 5 | 1 | 60 |
| 5 | 2 | 70 |
| 5 | 5 | 80 |
| 5 | 6 | 69 |
| 6 | 1 | 76 |
| 6 | 2 | 88 |
| 6 | 3 | 87 |
| 7 | 4 | 80 |
| 8 | 2 | 71 |
| 8 | 3 | 58 |
| 8 | 5 | 68 |
| 9 | 2 | 88 |
| 10 | 1 | 77 |
| 10 | 2 | 76 |
| 10 | 3 | 80 |
| 10 | 4 | 85 |
| 10 | 5 | 83 |
| 11 | 3 | 80 |
| 12 | 4 | 99 |
| 13 | 5 | 74 |
+------------+-----------+-------+
I want to show student_id and students' average scores that are higher than 80.
The output I want is like this:
+------------+-------------------+
| student_id | Average |
+------------+-------------------+
| 1 | 83.25 |
| 4 | 83.75 |
| 6 | 83.66666666666667 | // and how can I make this result shorter like 83.67?
| 7 | 80 |
| 9 | 88 |
| 10 | 80.2 |
| 11 | 80 |
| 12 | 99 |
+------------+-------------------+
I've tried the following codes
mysql> select student_id, avg(score) as average_score
-> from studentscore
-> group by student_id
-> where avg(score) >= 80;
and it gave me an syntax error.
I know by rules the where clause should go before the group by clause but I can't because the where clause depends on the result from the group by clause, and if I switch their position it will give me another error("Invalid use of group function").
Can some one tell me how to get the table I want?

Answer 1

use "having" instead of "where"

Answer 2

use having instead of where.
Here's the difference:
with where you can write a predicate that will be applied to each row
with having you can write a predicate that will applied to each group
and in your case, the 2nd is the only solution that can work.

Answer 3

select student_id, avg(score) as average_score
from studentscore
group by student_id
having avg(score) >= 80;

Answer 4

where applies a filter to your data before grouping has taken place, whereas having applies a filter post-grouping. round(,2) will format as you also ask:
select student_id, round(avg(score), 2) as average_score
from studentscore
group by student_id
having average_score >= 80;