How to do sum of different values without duplicate - postgresql

Question

How to do a sum of different values but same ID without duplicate different values on a column?
My Input in SQL Command.
SELECT
students.id AS student_id,
students.name,
COUNT(*) AS enrolled,
c2.price AS course_price,
(COUNT(*) * price) AS paid
FROM students
LEFT JOIN enrolls e on students.id = e.student_id
LEFT JOIN courses c2 on e.course_id = c2.id
WHERE student_id NOTNULL
GROUP BY students.id, students.name, c2.price
ORDER BY student_id ASC;
My result.
student_id | name | enrolled | paid
------------+---------------------+----------+------
1001 | Gulbadan Bálint | 1 | 90
1002 | Hanna Adair | 5 | 450
1003 | Taddeo Bhattacharya | 1 | 90
1004 | Persis Havlíček | 1 | 75
1004 | Persis Havlíček | 5 | 450
1005 | Tory Bateson | 1 | 90
1007 | Dávid Fèvre | 1 | 90
1008 | Masuyo Stoddard | 1 | 90
1009 | Iiris Levitt | 1 | 75
1009 | Iiris Levitt | 2 | 180
1013 | Artair Kovač | 1 | 30
1013 | Artair Kovač | 1 | 90
1015 | Matilda Guinness | 2 | 180
1017 | Margarita Ek | 1 | 90
1018 | Misti Zima | 3 | 270
1019 | Conall Ventura | 1 | 90
1020 | Vivian Monday | 2 | 180
My expected result.
student_id | name | enrolled | paid
------------+---------------------+----------+------
1001 | Gulbadan Bálint | 1 | 90
1002 | Hanna Adair | 5 | 450
1003 | Taddeo Bhattacharya | 1 | 90
1004 | Persis Havlíček | 6 | 525
1005 | Tory Bateson | 1 | 90
1007 | Dávid Fèvre | 1 | 90
1008 | Masuyo Stoddard | 1 | 90
1009 | Iiris Levitt | 3 | 255
1013 | Artair Kovač | 2 | 120
1015 | Matilda Guinness | 2 | 180
1017 | Margarita Ek | 1 | 90
1018 | Misti Zima | 3 | 270
1019 | Conall Ventura | 1 | 90
1020 | Vivian Monday | 2 | 180
I think that the cause come from a GROUP BY command but it will throw an error if I do not write a GROUP BY price.

Answer 1

Perhaps you can use SUM() function.
Please see link below, maybe it's same case with you:
how to group by and return sum row in Postgres

Answer 2

You have excluded course_price column both in your current and expected result. It seems you had wrongly included that in group by.
SELECT
students.id AS student_id,
students.name,
COUNT(*) AS enrolled,
--c2.price AS course_price, --exclude this in o/p?
(COUNT(*) * price) AS paid
FROM students
LEFT JOIN enrolls e on students.id = e.student_id
LEFT JOIN courses c2 on e.course_id = c2.id
WHERE student_id NOTNULL
GROUP BY students.id, students.name --,c2.price --and remove it from here
ORDER BY student_id ASC;