SQL Query to return 24 hour, hourly count even when no values exist?

Question

I've written a query that groups the number of rows per hour, based on a given date range.

SELECT CONVERT(VARCHAR(8),TransactionTime,101) + ' ' + CONVERT(VARCHAR(2),TransactionTime,108) as TDate, 
    COUNT(TransactionID) AS TotalHourlyTransactions
    FROM MyTransactions WITH (NOLOCK)
    WHERE TransactionTime BETWEEN CAST(@StartDate AS SMALLDATETIME) AND CAST(@EndDate AS SMALLDATETIME)
    AND TerminalId = @TerminalID
    GROUP BY CONVERT(VARCHAR(8),TransactionTime,101) + ' ' + CONVERT(VARCHAR(2),TransactionTime,108)
    ORDER BY TDate ASC

Which displays something like this:

02/11/20 07 4
02/11/20 10 1
02/11/20 12 4
02/11/20 13 1
02/11/20 14 2
02/11/20 16 3

Giving the number of transactions and the given hour of the day.

How can I display all hours of the day - from 0 to 23, and show 0 for those which have no values?

Thanks.

UPDATE

Using the tvf below works for me for one day, however I'm not sure how to make it work for a date range.

Using the temp table of 24 hours:

 -- temp table to store hours of the day    
 DECLARE @tmp_Hours TABLE ( WhichHour SMALLINT )

 DECLARE @counter SMALLINT
 SET @counter = -1
 WHILE @counter < 23 
    BEGIN
        SET @counter = @counter + 1
      --print 
        INSERT  INTO @tmp_Hours
                ( WhichHour )
        VALUES  ( @counter )
    END 

    SELECT MIN(CONVERT(VARCHAR(10),[dbo].[TerminalTransactions].[TransactionTime],101)) AS TDate, [@tmp_Hours].[WhichHour], CONVERT(VARCHAR(2),[dbo].[TerminalTransactions].[TransactionTime],108) AS TheHour,
        COUNT([dbo].[TerminalTransactions].[TransactionId]) AS TotalTransactions, 
        ISNULL(SUM([dbo].[TerminalTransactions].[TransactionAmount]), 0) AS TransactionSum
    FROM [dbo].[TerminalTransactions] RIGHT JOIN @tmp_Hours ON [@tmp_Hours].[WhichHour] = CONVERT(VARCHAR(2),[dbo].[TerminalTransactions].[TransactionTime],108) 
    GROUP BY [@tmp_Hours].[WhichHour], CONVERT(VARCHAR(2),[dbo].[TerminalTransactions].[TransactionTime],108),  COALESCE([dbo].[TerminalTransactions].[TransactionAmount], 0)

Gives me a result of:

TDate      WhichHour TheHour TotalTransactions TransactionSum
---------- --------- ------- ----------------- ---------------------
02/16/2010 0         00      4                 40.00
NULL       1         NULL    0                 0.00
02/14/2010 2         02      1                 10.00
NULL       3         NULL    0                 0.00
02/14/2010 4         04      28                280.00
02/14/2010 5         05      11                110.00
NULL       6         NULL    0                 0.00
02/11/2010 7         07      4                 40.00
NULL       8         NULL    0                 0.00
02/24/2010 9         09      2                 20.00

So how can I get this to group properly?

The other issue is that for some days there will be no transactions, and these days also need to appear.

Thanks.

Solution 1

So going back to using Remus' original function, I've re-used it in a recursive call and storing the results in a temp table:

DECLARE @count INT
DECLARE @NumDays INT
DECLARE @StartDate DATETIME
DECLARE @EndDate DATETIME
DECLARE @CurrentDay DATE

    DECLARE @tmp_Transactions TABLE 
    (
        StartHour DATETIME,
        TotalHourlyTransactions INT
    )   

SET @StartDate = '2000/02/10'
SET @EndDate = '2010/02/13'
SET @count = 0
SET @NumDays = DateDiff(Day, @StartDate, @EndDate)
WHILE @count < @NumDays 
    BEGIN
        SET @CurrentDay = DateAdd(Day, @count, @StartDate)
        INSERT INTO @tmp_Transactions (StartHour, TotalHourlyTransactions)
            SELECT  h.StartHour ,
                    t.TotalHourlyTransactions
            FROM    tvfGetDay24Hours(@CurrentDay) AS h
                    OUTER APPLY ( SELECT    COUNT(TransactionID) AS TotalHourlyTransactions
                                  FROM      [dbo].[TerminalTransactions]
                                  WHERE     TransactionTime BETWEEN h.StartHour AND h.EndHour
                                            AND TerminalId = 4
                                ) AS t
            ORDER BY h.StartHour
        SET @count = @Count + 1
    END 

SELECT *
FROM @tmp_Transactions

Solution 2

You do this by building first the 23 hours table, the doing an outer join against the transactions table. I use, for same purposes, a table valued function:

create function tvfGetDay24Hours(@date datetime)
returns table
as return (
select dateadd(hour, number, cast(floor(cast(@date as float)) as datetime)) as StartHour
  , dateadd(hour, number+1, cast(floor(cast(@date as float)) as datetime)) as EndHour
from master.dbo.spt_values
where number < 24 and type = 'p');

Then I can use the TVF in queries that need to get 'per-hour' basis data, even for missing intervals in the data:

select h.StartHour, t.TotalHourlyTransactions
from tvfGetDay24Hours(@StartDate) as h
outer apply (
  SELECT 
    COUNT(TransactionID) AS TotalHourlyTransactions
    FROM MyTransactions 
    WHERE TransactionTime BETWEEN h.StartHour and h.EndHour
    AND TerminalId = @TerminalID) as t
order by h.StartHour

Updated

Example of a TVF that returns 24hours between any arbitrary dates:

create function tvfGetAnyDayHours(@dateFrom datetime, @dateTo datetime)
returns table
as return (
select dateadd(hour, number, cast(floor(cast(@dateFrom as float)) as datetime)) as StartHour
  , dateadd(hour, number+1, cast(floor(cast(@dateFrom as float)) as datetime)) as EndHour
from master.dbo.spt_values
where type = 'p'
and number < datediff(hour,@dateFrom, @dateTo) + 24);

Note that since master.dbo.spt_values contains only 2048 numbers, the function will not work between dates further apart than 2048 hours.

Solution 3

You have just discovered the value of the NUMBERS table. You need to create a table with a single column containing the numbers 0 to 23 in it. Then you join again this table using an OUTER join to ensure you always get 24 rows returned.