SQL 表中的条件百分比列

Question

I want to add a column showing percentage values for Score, in which each record's score is divided by the sum of the scores for the category it's in. So the values in the new column should be 0.6, 0.4, 0.3 and 0.7.

Id   Category   Score
1    foo        6        
2    foo        4          
3    bar        30
4    bar        70

The best I can think to do is to create a temporary table that creates sum values for Score using GROUP BY and then JOIN-ing it to the main table. Is there a more efficient way of doing this?

Answer 1

Use window functions:

select t.*,
       score * 1.0 / sum(score) over (partition by category) as newcol
from t;

The * 1.0 is because some databases do integer division.

Answer 2

You can also do it without creating temporary table.

SELECT A.* , CAST(Score*1.0/TotalScore AS DECIMAL(6,2))
FROM [table1] A 
JOIN (
        SELECT      Category
                    ,SUM(Score) TotalScore 
        FROM        [table1] 
        GROUP BY    Category
      ) B
ON    A.Category = B.Category