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pivot (if else) - 행에 있던 값을 열로 회전해서 보는 것 (집계결과)
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-- job별 사람수
select job, count(empno)
from emp
group by job;


select 1 clerk, 1 salesman, 1 manager, 1 analyst, 1 president
from dual;

select case when job='clerk' then 1 else 0 end clerk, job
from emp;
