Question

I’ve been working on this for 2 days and I can’t figure it out. I’m hoping someone smarter than me will give this a go.

Let’s say I have the following tables:

Rating:
  Id | Name
  1  | A
  2  | B
  3  | C
  4  | D
  5  | E

Inspection:
  Id | Date (mm/dd/yyyy)
  1  | 01/04/2012
  2  | 04/04/2012
  3  | 28/03/2012
  4  | 04/04/2012

Observation:
  Id | InspectionId | RatingId
  1  | 2            | 3
  2  | 1            | 2
  3  | 4            | 3
  4  | 2            | 1
  5  | 3            | 3
  6  | 1            | 2

I want the query to return:

 RatingName | Date(mm/dd/yyyy) | ObservationCount
 A          | 01/04/2012       | 0
 B          | 01/04/2012       | 1
 C          | 01/04/2012       | 1
 D          | 01/04/2012       | 0
 E          | 01/04/2012       | 0
 A          | 04/04/2012       | 1
 B          | 04/04/2012       | 0
 C          | 04/04/2012       | 2
 D          | 04/04/2012       | 0
 E          | 04/04/2012       | 0
 A          | 28/03/2012       | 0
 B          | 28/03/2012       | 0
 C          | 28/03/2012       | 1
 D          | 28/03/2012       | 0
 E          | 28/03/2012       | 0

So I need the number of Observations for each rating for each date. And yes I need to have the records which return 0 Observations because I’m using this data in a stacked chart and without them it throws an error. I’ve managed to get the above table but without the records that return 0 Observations with the following Linq To Sql query, but from this point I get stuck.

MyDataContext DB = new MyDataContext();

var data = 
    (from r in DB.Ratings
    join o in DB.Observations on r.Id equals o.RatingId into ro

    from observation in ro.DefaultIfEmpty()
    join i in DB.Inspections on observation.InspectionId equals i.Id into roi

    from q in roi.DefaultIfEmpty()
    group q by new { Name = r.Name, Date = q.Date } into grouped
    select
    new
    {
        RatingName = grouped.Key.Name,
        Date = grouped.Key.Date,
        ObservationCount = grouped.Count(x => x != null)
    }).OrderBy(x => x.Date);

I would appreciate an answer in either Linq To Sql or just plain old SQL, thanks!

Answer 1

You should try a CROSS JOIN on the two reference tables, and then OUTER JOIN back to your ‘data’ table – and then check whether the Observation is null… then group and sum!

SELECT 
    [Name],
    [Date],
    SUM([Exists])
FROM
(
SELECT 
    name, 
    [Date],
    CASE WHEN o.Id IS NULL THEN 0
        ELSE 1
    END as [Exists]
FROM 
    Rating r CROSS JOIN
    Inspection i
    LEFT OUTER JOIN Observation o 
        ON o.RatingId = r.Id AND o.InspectionId = i.Id
) as [source]
GROUP BY [Name], [Date]
ORDER BY
    [Date],
    name

Translating to LINQ would be a similar two-step process – get the inner result set (checking whether observation is NULL), before grouping and summing.