Oracle error: ORA-00979: not a GROUP BY expression - sql

Question

Following is the result of my first query (in Oracle). Totals is the total number of employees in each department (so Employee 1-5 are on department#1, employees 6-8 on department#2 etc.)
select a.name, (select count(*) from employee b where a.dname = b.dname group by b.dname) as totals
from employee a;
NAME TOTALS
------------------------------ ----------------
Employee 1 5
Employee 2 5
Employee 3 5
Employee 4 5
Employee 5 5
Employee 6 3
Employee 7 3
Employee 8 3
Employee 9 4
Employee 10 4
Employee 11 4
Employee 12 4
And here is my second query on which employer has less taskhours than 5 (note "taskhours" is a different field than "totals" in the database)
select a.name
from job o, employee a, works w , task t
where w.name=a.name and w.menuid=o.menuid and w.worktype='office' and t.fname = o.fname
group by a.name
having sum(t.taskhours) <5
;
NAME
------------------------------
Employee 3
I want now combine the above, i.e query for the employees who have fewer task-hours than the total employees in the department. I am trying this query
select a.name
from job o, employee a, works w , task t
where w.name=a.name and w.menuid=o.menuid and w.worktype='office' and t.fname = o.fname
group by a.name
having sum(t.taskhours) <(select count(*) from employee b where a.sname = b.sname group by b.sname)
but I get the error:
ERROR at line 5:
ORA-00979: not a GROUP BY expression
Can someone help me find the right query?
Sample data for 1st query:
NAME DNAME
------------------------------ ------------------------------
Employee 1 Dep1
Employee 2 Dep1
Employee 3 Dep1
Employee 4 Dep1
Employee 5 Dep1
Employee 6 Dep2
Employee 7 Dep2
Employee 8 Dep2
Employee 9 Dep3
Employee 10 Dep3
Employee 11 Dep3
Employee 12 Dep3

Answer 1

We can phrase this by putting each of your two queries into subqueries, and then joining them together:
SELECT
e.name,
e.dname,
t1.emp_cnt,
t2.hours_cnt
FROM employee e
LEFT JOIN
(
SELECT dname, COUNT(*) AS emp_cnt
FROM employee
GROUP BY dname
) t1
ON e.dname = t1.dname
LEFT JOIN
(
SELECT a.name, SUM(t.taskhours) AS hours_cnt
FROM works w
INNER JOIN employee a
ON w.name = a.name
INNER JOIN job o
ON w.menuid = o.menuid
INNER JOIN task t
ON t.fname = o.fname
WHERE w.worktype = 'office'
GROUP BY a.name
) t2
ON e.name = t2.name
WHERE
t1.emp_cnt < t2.hours_cnt;

Answer 2

You can simplify the first query as follows
use that as a cte block
with tot_count
as (select dept_id,count(emp_id) as dept_cnt
from employee
group by dept_id)
,less_than_five
as (select a.name
,sum(t.taskhours) as sum_taskhours
,max(a.dept_id) as dept_id_of_emp
from job o,
employee a,
works w ,
task t
where w.name=a.name
and w.menuid=o.menuid
and w.worktype='office'
and t.fname = o.fname
group by a.name
)
select *
from less_than_five a
join tot_count b
on a.dept_id_of_emp=b.dept_id
where less_than_five.sum_taskhours<=b.dept_cnt