Question

I am a new developer in ASP.NET with C#. I developed a web-based application for one of the companies, and now I am required to develop a log that shows the number of unique people accessing the system. For example, Employees A and B from Division AA accessed the system today. Employee A accessed the system twice at the same day. The log should show that there are two employees from Division AA accessed the system today not three.

I have the following database design:

Employee Table: Username, Name, DivisionCode (DivisionCode is a foreign key to SapCode)
Divisions Table: SapCode DivisionShortcut   
Log Table: ID, Username, DateTimeAccessing

I wrote the following query that shows the username, employee name, division and DateTimeAccessing:

SELECT     dbo.[Log].Username, dbo.employee.Name, dbo.Divisions.DivisionShortcut, dbo.[Log].DateTimeAccessing
FROM         dbo.employee INNER JOIN
                      dbo.[Log] ON dbo.employee.Username = dbo.[Log].Username INNER JOIN
                      dbo.Divisions ON dbo.employee.DivisionCode = dbo.Divisions.SapCode

The query should show the number of employees who accessed the system everyday based on division and date such as:

At Nove 21, Division AA = 2    
   Nove 21, Division BB = 7    
   Nove 20, Division AA = 12 and so on

So how to do that?

*UPDATE:*
I am using now the following query:

SELECT     CONVERT(varchar(10), l.DateTimeAccessing, 120) AS Date, d.DivisionShortcut, COUNT(DISTINCT l.Username) AS Cnt
FROM         dbo.employee AS e INNER JOIN
                      dbo.[Log] AS l ON e.Username = l.Username RIGHT OUTER JOIN
                      dbo.Divisions AS d ON e.DivisionCode = d.SapCode
GROUP BY CONVERT(varchar(10), l.DateTimeAccessing, 120), d.DivisionShortcut

The problem now is for example if I want to see the statistics for today or Nov 10 for all divisions, the date column will show me NULL values for the division which have no employees accessing the system at that date. Why? And how to fix it?

Answer 1

Use count(distinct) to count the number of users, and group by to group the result on divisons and dates.

You can use convert to get the date as a string from the datetime, but be aware that it slows down the query a bit.

select d.DivisionShortcut, convert(varchar(10),l.DateTimeAccessing,120), count(distinct l.Username) as Cnt
from dbo.employee e
inner join dbo.[Log] l on e.Username = l.Username
inner join dbo.Divisions d on e.DivisionCode = d.SapCode
group by d.DivisionShortcut, convert(varchar(10),l.DateTimeAccessing,120)

Update:

Here is how you can join in the dates:

select d.DivisionShortcut, t.DateAccessing, count(distinct l.Username) as Cnt
from dbo.employee e
inner join dbo.[Log] l on e.Username = l.Username
inner join dbo.Divisions d on e.DivisionCode = d.SapCode
inner join (
  select distinct convert(varchar(10),DateTimeAccessing,120) as DateAccessing
  from dbo.Divisions
) t on t.DateAccessing = convert(varchar(10),l.DateTimeAccessing,120)
group by d.DivisionShortcut, t.DateAccessing