Question

I want to fill in the missing months in a SELECT statement so I was planning on joining my table with another table that contains all months. How can I generate a table of months in a light-weight fashion? For instance,

CREATE TABLE #TEMP(Timewhen DATETIME, Value INT)

INSERT INTO #TEMP VALUES('2012-02-04', 4)
INSERT INTO #TEMP VALUES('2012-02-06', 4)
INSERT INTO #TEMP VALUES('2012-02-10', 4)
INSERT INTO #TEMP VALUES('2012-04-08', 4)
INSERT INTO #TEMP VALUES('2012-04-12', 4)

SELECT YEAR(Timewhen) EventYear, MONTH(Timewhen) EventMonth, SUM(Value) Total
FROM #TEMP
GROUP BY YEAR(Timewhen), MONTH(Timewhen)

DROP TABLE #TEMP

gives me:

EventYear   EventMonth  Total
2012               2    12
2012               4    8

But I need:

EventYear   EventMonth  Total
2012        2           12
2012        3           0
2012        4           8

Answer 1

DECLARE @months     INT, 
        @firstmonth DATE;

SELECT 
    @months     = DATEDIFF(MONTH, MIN(Timewhen), MAX(Timewhen)) + 1, 
    @firstmonth = DATEADD(DAY, 1-DAY(MIN(Timewhen)), MIN(Timewhen))
FROM #temp;

;WITH m(rn) AS 
(
  SELECT TOP (@months) rn = ROW_NUMBER() OVER (ORDER BY [object_id])
  FROM sys.objects ORDER BY rn
),
x(d) AS 
(
  SELECT DATEADD(MONTH, rn-1, @firstmonth) FROM m
)
SELECT YEAR(x.d), MONTH(x.d), Total = SUM(COALESCE(t.Value, 0))
  FROM x 
  LEFT OUTER JOIN #temp AS t
  ON t.Timewhen >= x.d AND t.Timewhen < DATEADD(MONTH, 1, x.d)
  GROUP BY YEAR(x.d), MONTH(x.d);

Or a slightly less verbose version:

DECLARE @months     INT, 
        @firstmonth DATE;

SELECT 
    @months     = DATEDIFF(MONTH, MIN(Timewhen), MAX(Timewhen)) + 1, 
    @firstmonth = DATEADD(DAY, 1-DAY(MIN(Timewhen)), MIN(Timewhen))
FROM #temp;

;WITH x(y, m, s, e) AS 
(
  SELECT YEAR(dt), MONTH(dt), dt, DATEADD(MONTH, 1, dt) FROM 
  ( SELECT dt = DATEADD(MONTH, rn-1, @firstmonth) FROM 
    ( SELECT TOP (@months) rn = ROW_NUMBER() OVER (ORDER BY [object_id])
      FROM sys.objects ORDER BY rn
    ) AS z
  ) AS y
)
SELECT EventYear = x.y, EventMonth = x.m, Total = SUM(COALESCE(t.Value, 0))
  FROM x LEFT OUTER JOIN #temp AS t
  ON t.Timewhen >= x.s AND t.Timewhen < x.e
  GROUP BY x.y, x.m;

Alternate solution using Calendar table:

Create a Calendar table using instructions from here.

-- Script to create a calendar table

DROP TABLE dbo.Numbers
DROP TABLE dbo.Calendar
GO

-- Use this to determine the number in the next query
DECLARE @NUMDAYS int
SELECT @NUMDAYS = DATEDIFF(DAY, '20000101', '20291231')

CREATE TABLE dbo.Numbers 
( 
    Number INT IDENTITY(1,1) PRIMARY KEY CLUSTERED 
) 

WHILE COALESCE(SCOPE_IDENTITY(), 0) <= @NUMDAYS
BEGIN 
    INSERT dbo.Numbers DEFAULT VALUES 
END
GO

CREATE TABLE dbo.Calendar  
(  
    dt SMALLDATETIME NOT NULL 
        PRIMARY KEY CLUSTERED,  

    isWeekday BIT, 
    isHoliday BIT,  
    Y SMALLINT,  
    FY SMALLINT,  
    Q TINYINT,  
    M TINYINT,  
    D TINYINT,  
    DW TINYINT, 
    monthname VARCHAR(9), 
    dayname VARCHAR(9), 
    W TINYINT ,
    HolidayDescription VARCHAR(32)
) 
GO



INSERT Calendar(dt) 
SELECT DATEADD(DAY, Number, '20000101') 
FROM dbo.Numbers 
--WHERE Number <= @NUMDAYS 
ORDER BY Number

GO

--SELECT * FROM Calendar

UPDATE dbo.Calendar SET 

    isWeekday = CASE  
        WHEN DATEPART(DW, dt) IN (1,7)  
        THEN 0 
        ELSE 1 END, 

    isHoliday = 0, 

    Y = YEAR(dt),  

    FY = YEAR(dt), 

    /* 
    -- if our fiscal year 
    -- starts on May 1st: 

    FY = CASE  
        WHEN MONTH(dt) < 5 
        THEN YEAR(dt)-1  
        ELSE YEAR(dt) END, 
    */ 

    Q = CASE 
        WHEN MONTH(dt) <= 3 THEN 1 
        WHEN MONTH(dt) <= 6 THEN 2 
        WHEN MONTH(dt) <= 9 THEN 3 
        ELSE 4 END,  

    M = MONTH(dt),  

    D = DAY(dt),  

    DW = DATEPART(DW, dt),  

    monthname = DATENAME(MONTH, dt),  

    dayname = DATENAME(DW, dt),  

    W = DATEPART(WK, dt)

GO

After creating a Calendar table, one can use the following to achieve this:

CREATE TABLE #TEMP(Timewhen DATETIME, Value INT)

INSERT INTO #TEMP VALUES('2012-02-04', 4)
INSERT INTO #TEMP VALUES('2012-02-06', 4)
INSERT INTO #TEMP VALUES('2012-02-10', 4)
INSERT INTO #TEMP VALUES('2012-04-08', 4)
INSERT INTO #TEMP VALUES('2012-04-12', 4)

SELECT Y EventYear, M EventMonth, SUM(Value) Total
FROM #TEMP RIGHT OUTER JOIN (SELECT DISTINCT Y,M FROM dbo.dateRange('20120204', '20120412')) X
ON YEAR(Timewhen) = X.Y AND MONTH(Timewhen) = X.M
GROUP BY Y,M

DROP TABLE #TEMP