Question

I have the following database design:

Employees Table: EmployeeID, Name, OrgCode
Departments Table: OrgCode, DepartName
CompleteSurvey Table: ID, ParticipantID

And I need to develop one query that will display a table that shows the total number of employees in all departments and the total number of participants who completed the survey in all departments.

I could be able to find the total number of employees in all department by the following query:

SELECT     COUNT(DISTINCT dbo.Employees.EmployeeID) AS [Total Number of Employees]
FROM         dbo.Departments INNER JOIN
                      dbo.Employees ON dbo.Departments.OrgCode = dbo.Employees.OrgCode CROSS JOIN
                      dbo.CompleteSurvey

Then, I could be able to find the total number of participants in all department by the following query:

SELECT     COUNT(DISTINCT dbo.CompleteSurvey.ID) AS [Total Number of Participants]
FROM         dbo.Departments INNER JOIN
                      dbo.Employees ON dbo.Departments.OrgCode = dbo.Employees.OrgCode INNER JOIN
                      dbo.CompleteSurvey ON dbo.Employees.EmployeeID = dbo.CompleteSurvey.RespondantID

But I should have a one query only.

For example, if Department A has 100 employee and the number of participants is 50 out of 100
and Department B has 80 employee and the number of participants is 30

The query should show the following:

the total number of employees in all departments = 180

the total number of participants in all departments = 80

the percent completion in all of them = 80/180 = 44%

So how to do that?

Answer 1

You don’t need to involve the Department table in these calculations:

SELECT
  TotalEmployees          = COUNT(*),
  TotalParticipants       = COUNT(s.ParticipantID),
  ParticipationPercentage = COUNT(s.ParticipantID) * 100.0 / COUNT(*)
FROM dbo.Employee e
  LEFT JOIN (SELECT DISTINCT ParticipantID FROM dbo.CompleteSurvey) s
    ON e.EmployeeID = s.ParticipantID