a query displaying 0 instead of not showing - sql

Question

the following query is displaying the result i want except i want it to show 0 for each month with non production.
SELECT
DATENAME(MONTH, DATEADD(M, MONTH(PolicyDetails.IssuedDate), - 1)) AS Month,
SUM(PolicyDetails.Premium) AS TotalProduction,
DATENAME(YEAR, PolicyDetails.IssuedDate) AS Year
FROM PolicyDetails INNER JOIN Clients
ON PolicyDetails.ClientId = Clients.ClientId
WHERE (Clients.Username = #Username)
GROUP BY MONTH(PolicyDetails.IssuedDate), DATENAME(YEAR, PolicyDetails.IssuedDate)
Month Total Production -$$
2019 - August 30.00
2019 - October 45.00
in this table i want to show "2019 - September" with Total Production = 0 instead of displaying nothing. How ??

Answer 1

If you want to show all months in the data, probably the simplest method is to use conditional aggregation. Your calculation of the month seems awkward. You seem to want the previous month, so:
SELECT DATENAME(YEAR, pd.IssuedDate) AS Year,
DATENAME(MONTH, DATEADD(MONTH, -1, pd.IssuedDate)) AS Month,
SUM(CASE WHEN c.Username = #Username THEN pd.Premium ELSE 0 END) AS TotalProduction
FROM PolicyDetails pd INNER JOIN
Clients c
ON pd.ClientId = c.ClientId
GROUP BY DATENAME(YEAR, pd.IssuedDate), DATENAME(MONTH, DATEADD(MONTH, -1, pd.IssuedDate))
ORDER BY MIN(pd.IssuedDate)
This assumes that you have at least one row per month in the data.
Otherwise, the canonical approach is to generate the months you want (using a derived table or recursive CTE or calendar table). Your month arithmetic is a bit awkward for that solution. It would look like:
SELECT YEAR(DATEADD(MONTH, -1, months.mstart)),
MONTH(DATEADD(MONTH, -1, months.mstart)),
COALESCE(SUM(pd.Premium), 0) AS TotalProduction
FROM (VALUES (CONVERT(DATE, '2019-08-01')),
(CONVERT(DATE, '2019-09-01')),
(CONVERT(DATE, '2019-10-01'))
) months(mstart) LEFT JOIN
PolicyDetails pd
ON pd.IssuedDate >= DATEADD(month, -1, months.mstart) AND
pd.IssuedDate < months.mstart LEFT JOIN
Clients c
ON pd.ClientId = c.ClientId AND
c.Username = #Username
GROUP BY YEAR(DATEADD(MONTH, -1, months.mstart)),
MONTH(DATEADD(MONTH, -1, months.mstart))
ORDER BY MIN(pd.IssuedDate)

Answer 2

I suppose that might be a usecase for SQL-IF.
Syntax as follows:
SELECT IF([condition], "YES", "NO");
In your case, try to combine that with your query.
Maybe like this:
SELECT IF([subquery > 0] AS X, X, 0);
If-Condition Like:
SELECT IF((SELECT COUNT(*) FROM PolicyDetails ) > 0, 1, 0);
But you will have to combine that output with your existing query posted above, to gain the output if the table is filled.