Question

I’m using SQL Server 2012 and have a database containing membership information. I’m trying to write SQL that will count the members of each type that exist during each month so I can report on if membership is going up/down month by month. A simplified version of my schema is:

http://www.sqlfiddle.com/#!6/5fed2/1

CREATE TABLE MemberType
(
    MemberTypeID INT,
    Name VARCHAR(50)
);

CREATE TABLE Person
(
    PersonID INT,
    Name VARCHAR(50)
);

CREATE TABLE Invoice
(
    InvoiceID INT,
    DateStart DATE,
    DateEnd   DATE,
    PersonID  INT,     --(FK) Person that purchased the membership
    MemberTypeID INT,  --(FK) Type of membership
);

INSERT MemberType (MemberTypeID,Name) VALUES (1,'Regular')
INSERT MemberType (MemberTypeID,Name) VALUES (2,'Special')
INSERT MemberType (MemberTypeID,Name) VALUES (3,'Honorary')

INSERT Person (PersonID,Name) VALUES (1,'Joe Smith')
INSERT Person (PersonID,Name) VALUES (2,'Anna Smith')
INSERT Person (PersonID,Name) VALUES (3,'Ted Williams')
INSERT Person (PersonID,Name) VALUES (4,'Bill Williams')

INSERT Invoice VALUES(1,'2011-5-1', '2012-1-1', 1, 1)
INSERT Invoice VALUES(2,'2010-1-1', '2013-1-1', 2, 2)
INSERT Invoice VALUES(3,'2012-2-1', '2013-2-1', 3, 1)
INSERT Invoice VALUES(4,'2009-1-1', '2011-5-1', 1, 1)
INSERT Invoice VALUES(5,'2011-1-1', '2012-5-1', 4, 1)

I’ve tried to get the member counts for a specific date like this:

DECLARE @RunDt DATE='2011-1-1'

SELECT
mt.Name [MemberType], @RunDt [Date], count(p.PersonID) [Count]
FROM MemberType mt
LEFT OUTER JOIN Invoice i ON i.MemberTypeID=mt.MemberTypeID
LEFT OUTER JOIN Person p ON p.PersonID=i.PersonID
WHERE @RunDt BETWEEN i.DateStart AND i.DateEnd
GROUP BY mt.Name

I think this is close but I want the honorary type to be included with a count of 0. I thought the LEFT OUTER JOINs would accomplish that, but thy do not.
Also this is only for a single date. My next step is to get the results of that query for the 1st of each month all together in a single result set.

Answer 1

By placing the filter criteria for the Invoice table in the WHERE clause, you’ve essentially converted your OUTER JOIN to an INNER JOIN. Try putting that filter criteria in the ON criteria for the OUTER JOIN against Invoice.

SELECT 
  mt.Name AS [MemberType], 
  @RunDt AS [Date], 
  count(p.PersonID) AS [Count]
FROM dbo.MemberType AS mt
LEFT OUTER JOIN dbo.Invoice AS i 
  ON i.MemberTypeID=mt.MemberTypeID
  AND @RunDt BETWEEN i.DateStart AND i.DateEnd
LEFT OUTER JOIN dbo.Person AS p 
  ON p.PersonID = i.PersonID
GROUP BY mt.Name;

Demo:

http://www.sqlfiddle.com/#!6/5fed2/2

---

If you want to do this for every month in a given year:

DECLARE @year INT = 2012;

;WITH x(d) AS 
(
    SELECT CONVERT(DATE, DATEADD(MONTH, n-1, 
      DATEADD(YEAR, @year-1900, '19000101')))
    FROM 
    (
      SELECT TOP (12) n = ROW_NUMBER() OVER 
        (ORDER BY [object_id])
      FROM sys.all_objects 
      ORDER BY [object_id]
    ) AS y
)
SELECT
    mt.[Name],
    [Date] = x.d, 
    [Count] = COUNT(p.PersonID)
FROM x CROSS JOIN dbo.MemberType AS mt
LEFT OUTER JOIN dbo.Invoice AS i 
ON i.MemberTypeID = mt.MemberTypeID
AND x.d BETWEEN i.DateStart AND i.DateEnd
LEFT OUTER JOIN dbo.Person AS p 
ON p.PersonID = i.PersonID
GROUP BY mt.Name, x.d
ORDER BY x.d, mt.Name;

SQLFiddle demo:

http://sqlfiddle.com/#!6/f5f84/2

More info on generating sets without loops:

http://www.sqlperformance.com/2013/01/t-sql-queries/generate-a-set-1
http://www.sqlperformance.com/2013/01/t-sql-queries/generate-a-set-2
http://www.sqlperformance.com/2013/01/t-sql-queries/generate-a-set-3