How to get distinct employees that do not have a particular skillset - sql

Question

I have a table that has two columns. Employee_id (which is unique per employee) and next column for employee skillset. One employee can have multiple skillset. How do I retrieve the list of distinct employees who don't have skillset 'c' if A,B,C,D,E are the five types of skillset that employees can have.
employee_id skillset
1 A
1 C
2 E
3 A
3 B
3 C
4 D
4 C
5 B
I have tried self join and other methods but it is not working.
select distinct employee_id from employee_skillset where skillset not like 'C'
When I run my query, it is still giving me employee_ids that have skillset of "c"

Answer 1

You can group by employee_id and set a condition in the HAVING clause:
select employee_id
from employee_skillset
group by employee_id
having sum(case when skillset = 'C' then 1 else 0 end) = 0
Or with NOT EXISTS:
select distinct s.employee_id
from employee_skillset s
where not exists (
select 1 from employee_skillset
where employee_id = s.employee_id and skillset = 'C'
)

Answer 2

What are your expected results from your data set? 2 and 5?
Why not something like below
SELECT DISTINCT employee_id
FROM Table1
WHERE skillset <> 'C';

Answer 3

MINUS set operator is one option:
SQL> with employee_skillset (employee_id, skillset) as
2 (select 1, 'a' from dual union all
3 select 1, 'c' from dual union all
4 select 2, 'e' from dual union all
5 select 3, 'a' from dual union all
6 select 3, 'b' from dual union all
7 select 3, 'c' from dual union all
8 select 4, 'd' from dual union all
9 select 4, 'c' from dual union all
10 select 5, 'b' from dual
11 )
12 select employee_id from employee_skillset
13 minus
14 select employee_id from employee_skillset where skillset = 'c';
EMPLOYEE_ID
-----------
2
5
SQL>
Yet another option:
<snip>
12 select employee_id
13 from (select employee_id,
14 case when skillset = 'c' then 1 else 0 end flag
15 from employee_skillset
16 )
17 group by employee_id
18 having sum(flag) = 0;
EMPLOYEE_ID
-----------
2
5
SQL>
Or:
<snip>
12 select employee_id
13 from (select employee_id,
14 listagg(skillset, ',') within group (order by null) lagg
15 from employee_skillset
16 group by employee_id
17 )
18 where instr(lagg, 'c') = 0;
EMPLOYEE_ID
-----------
2
5
SQL>