How to calculate percentage with a SQL statement

1. The most efficient (using over()).
   

   select Grade, count(*) * 100.0 / sum(count(*)) over()
   from MyTablegroup by Grade

2. Universal (any SQL version).
   

   select Rate, count(*) * 100.0 / (select count(*) from MyTable)
   from MyTablegroup by Rate;
   

3. With CTE, the least efficient.
   

   with t(Rate, RateCount) as ( 
       select Rate, count(*) 
       from MyTable
       group by Rate)
   select Rate, RateCount * 100.0/(select sum(RateCount) from t)
   from t;
   

http://stackoverflow.com/questions/770579/how-to-calculate-percentage-with-a-sql-statement