R, Group By in Subquery - group-by

Question

I was practicing on some subqueries and I got stuck on a problem. This is for the table below (snippet). The question is "From the following tables, write a SQL query to find those employees whose salaries exceed 50% of their department's total salary bill. Return first name, last name."
My query is this below, but it does not run. I ran the subquery by itself, and it ran fine. I think it's something to do with the GROUP BY in the subquery.
SELECT first_name, last_name
FROM employees
WHERE salary >
(
SELECT (sum(salary)) / 2
FROM employees
GROUP BY department_id
)
The correct answer from the practice is below. Is creating table e2 necessary?
SELECT e1.first_name, e1.last_name
FROM employees e1
WHERE salary >
( SELECT (SUM(salary))*.5
FROM employees e2
WHERE e1.department_id=e2.department_id);
+-------------+-------------+-------------+----------+--------------------+------------+------------+----------+----------------+------------+---------------+
| EMPLOYEE_ID | FIRST_NAME | LAST_NAME | EMAIL | PHONE_NUMBER | HIRE_DATE | JOB_ID | SALARY | COMMISSION_PCT | MANAGER_ID | DEPARTMENT_ID |
+-------------+-------------+-------------+----------+--------------------+------------+------------+----------+----------------+------------+---------------+
| 100 | Steven | King | SKING | 515.123.4567 | 2003-06-17 | AD_PRES | 24000.00 | 0.00 | 0 | 90 |
| 101 | Neena | Kochhar | NKOCHHAR | 515.123.4568 | 2005-09-21 | AD_VP | 17000.00 | 0.00 | 100 | 90 |
| 102 | Lex | De Haan | LDEHAAN | 515.123.4569 | 2001-01-13 | AD_VP | 17000.00 | 0.00 | 100 | 90 |
| 103 | Alexander | Hunold | AHUNOLD | 590.423.4567 | 2006-01-03 | IT_PROG | 9000.00 | 0.00 | 102 | 60 |
| 104 | Bruce | Ernst | BERNST | 590.423.4568 | 2007-05-21 | IT_PROG | 6000.00 | 0.00 | 103 | 60 |
| 105 | David | Austin | DAUSTIN | 590.423.4569 | 2005-06-25 | IT_PROG | 4800.00 | 0.00 | 103 | 60 |
| 106 | Valli | Pataballa | VPATABAL | 590.423.4560 | 2006-02-05 | IT_PROG | 4800.00 | 0.00 | 103 | 60 |
| 107 | Diana | Lorentz | DLORENTZ | 590.423.5567 | 2007-02-07 | IT_PROG | 4200.00 | 0.00 | 103 | 60 |
| 108 | Nancy | Greenberg | NGREENBE | 515.124.4569 | 2002-08-17 | FI_MGR | 12008.00 | 0.00 | 101 | 100 |
| 109 | Daniel | Faviet | DFAVIET | 515.124.4169 | 2002-08-16 | FI_ACCOUNT | 9000.00 | 0.00 | 108 | 100 |
| 110 | John | Chen | JCHEN | 515.124.4269 | 2005-09-28 | FI_ACCOUNT | 8200.00 | 0.00 | 108 | 100
SELECT first_name, last_name
FROM employees
WHERE salary >
(
SELECT (sum(salary)) / 2
FROM employees
GROUP BY department_id
)
I expected this to run, but it did not execute. The editor on the website I'm practicing from does not give error info.