How to return a default value when no rows are returned from the select statement - sql

Question

I have a select statement that returns two columns, a date column, and a count(value) column. When the count(value) column doesn't have any records, I need it to return a 0. Currently, it just skips that date record all together.
Here is the basics of the query.
select convert(varchar(25), DateTime, 101) as recordDate,
count(Value) as recordCount
from History
where Value < 700
group by convert(varchar(25), DateTime, 101)
Here are some results that I'm getting.
+------------+-------------+
| recordDate | recordCount |
+------------+-------------+
| 02/26/2014 | 143 |
| 02/27/2014 | 541 |
| 03/01/2014 | 21 |
| 03/02/2014 | 60 |
| 03/03/2014 | 113 |
+------------+-------------+
Notice it skips 2/28/2014. This is because the count(value) column doesn't have anything to count. How can I add the record in there that has the date of 2/28/2014, with a recordCount of 0?

Answer 1

To generate rows for missing dates you can join your data to a date dimension table
It would look something like this:
select convert(varchar(25), ddt.DateField, 101) as recordDate,
count(t.Value) as recordCount
from History h
right join dbo.DateDimensionTable ddt
on ddt.DateField = convert(varchar(25), h.DateTime, 101)
where h.Value < 700
group by convert(varchar(25), h.DateTime, 101)
If your table uses the DateTime column to store dates only (meaning the time is always midnight), then you can replace this
right join dbo.DateDimensionTable ddt
on ddt.DateField = convert(varchar(25), h.DateTime, 101)
with this
right join dbo.DateDimensionTable ddt
on ddt.DateField = h.DateTime

Answer 2

You may use COUNT(*). It will return zero if nothing was found for the column. Also you may group result set by value column if it is needed.

Answer 3

select convert(varchar(25), DateTime, 101) as recordDate,
CASE WHEN count(value) =0 THEN 0 ELSE COUNT(value) END recordCount
from History
where Value < 700
group by convert(varchar(25), DateTime, 101)

Answer 4

When you use a group by, it only creates a distinct list of values that exist in your records. Since 20140228 has no records, it will not show up in the group by.
Your best bet is to generate a list of values, dates in your case, and left join or apply that table against your history table.
I can't seem to copy my T-SQL in here so here's a hastebin.
http://hastebin.com/winaqutego.vbs

Answer 5

The best practice would be for you to have a datamart where a separate dimensional table for dates is kept with all dates you might be interested at - even if they lack amounts. DMason's answer shows the query with such a dimensional table.
To keep with the best practices you would have a fact table where you'd keep these historical data already pre-grouped at the granularity level you need (daily, in this case), so you wouldn't need a GROUP BY unless you needed a coarser granularity (weekly, monthly, yearly).
And in both your operational and datamart databases the dates would be stored as dates, not...
But then, since this is real world and you might not be able to change what somebody else made... If you: a) only care about the dates that appear in [History], and b) such dates are never stored with hours/minutes; then following query might be what you'd need:
SELECT MyDates.DateTime, COUNT(*)-1 AS RecordCount
FROM (
SELECT DISTINCT DateTime FROM History
) MyDates
LEFT JOIN History H
ON MyDates.DateTime = H.Datetime
AND H.Value < 700
GROUP BY MyDates.DateTime
Do try to add an index over DateTime and to further constrain the query with an earliest/latest date for better performance results.

Answer 6

I agree that a Dates table (AKA time dimension) is the right solution, but there is a simpler option:
SELECT
CONVERT(VARCHAR(25), DateTime, 101) AS RecordDate,
SUM(CASE WHEN Value < 700 THEN 1 ELSE 0 END) AS RecordCount
FROM
History
GROUP BY
CONVERT(VARCHAR(25), DateTime, 101)

Answer 7

Try this:
DECLARE #Records TABLE (
[RecordDate] DATETIME,
[RecordCount] INT
)
DECLARE #Date DATETIME = '02/26/2014' -- Enter whatever date you want to start with
DECLARE #EndDate DATETIME = '03/31/2014' -- Enter whatever date you want to stop
WHILE (1=1)
BEGIN
-- Insert the date into the temp table along with the count
INSERT INTO #Records (RecordDate, RecordCount)
VALUES (CONVERT(VARCHAR(25), #Date, 101),
(SELECT COUNT(*) FROM dbo.YourTable WHERE RecordDate = #Date))
-- Go to the next day
#Date = DATEADD(d, 1, #Date)
-- If we have surpassed the end date, break out of the loop
IF (#Date > #EndDate) BREAK;
END
SELECT * FROM #Records
If your dates have time components, you would need to modify this to check for start and end of day in the SELECT COUNT(*)... query.