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Como calcular porcentagem com uma instrução SQL


  1. O mais eficiente (usando over()). 
    select Grade, count(*) * 100.0 / sum(count(*)) over()
from MyTable
group by Grade

  2. Universal (qualquer versão SQL). 
    select Grade, count(*) * 100.0 / (select count(*) from MyTable)
from MyTable
group by Grade;

  3. Com CTE, o menos eficiente. 
    with t(Grade, GradeCount) 
as 
( 
    select Grade, count(*) 
    from MyTable
    group by Grade
)
select Grade, GradeCount * 100.0/(select sum(GradeCount) from t)
from t;