Column merge using sum in case Oracle APEX

I need help How can I merge the column into a single column, here is my code, is this method is correct. I want to get the count of the selected row in the table for the columns.

SELECT
    CAT_MGR,
    SUM  ( case  when CAT_MGR = 'A' THEN 1    else 0 end ) AS DESIGN,
          sum (case  when CAT_MGR = 'b' THEN 1 else 0  END) AS DESIGN,
          sum (case  when CAT_MGR = 'c' THEN 1 else 0  END) AS DESIGN
 from Table_A
 GROUP BY
  CAT_MGR

Can you guys help me I’m a beginner at SQL.

Thank you in advance

Answer

If you want just one row in the resultset, then remove the group by clause. Then, if you want to count the three cat mgr together, you can use in:

select
    sum(case when cat_mgr = 'a' then 1 else 0 end ) as design_a,
    sum(case when cat_mgr = 'b' then 1 else 0 end ) as design_b,
    sum(case when cat_mgr = 'c' then 1 else 0 end ) as design_c,
    sum(case when cat_mgr in ('a', 'b', 'c') then 1 else 0 end ) as design
 from Table_a