Question

In my schema, I have a table Projects, and a table Tasks. Each project is comprised of tasks. Each task has Hours and PercentComplete.

Example table:

ProjectID    TaskID    Hours   PercentComplete
1            1         100     50
1            2         120     80

I am trying to get the weighted percentage complete for the project. I am doing this using the following SQL statement:

SELECT P.ProjectID, P.ProjectName, SUM(T.Hours) AS Hours, 
SUM(T.PercentComplete * T.Hours) / 100 AS CompleteHours, 
SUM(T.PercentComplete * T.Hours) / SUM(T.Hours) AS PercentComplete
FROM Projects AS P INNER JOIN
        Tasks AS T ON T.ProjectID = P.ProjectID
WHERE     (P.ProjectID = 1)

My question is about this part of that statement:

SUM(T.PercentComplete * T.Hours) / SUM(T.Hours) AS PercentComplete

This gives me the correct weighted percentage for this project (in the case of the sample data above, 66%). But I cannot seem to wrap my head around why it does this.

Why does this query work?

Answer 1

• SUM(T.PercentComplete * T.Hours) / 100 is the number of complete hours.
• SUM(T.Hours) is the total number of hours.

• The ratio of these two amounts, i.e.:

  (SUM(T.PercentComplete * T.Hours) / 100) / SUM(T.Hours)
  

  is the proportion of hours complete (it should be between 0 and 1).

• Multiplying this by 100 gives the percentage.

I prefer to keep percentages like this out of the database and move them to the presentation layer. It would be much easier if the database stored “hours completed” and “hours total” and did not store the percentages at all. The extra factors of 100 in the calculations confuse the issue.