Group by year month sql, when no entry write 0 - sql

Question

I have an sql query, but I want to show months, where no entries. Now just show entries when it has entry. Here is the code:
SELECT YEAR(T0.[Recontact]) AS 'Év', MONTH(T0.[Recontact]) AS 'Hónap',
T1.[SlpName], COUNT(T0.[ClgCode]) AS 'Tárgyalások'
FROM OCLG T0
INNER JOIN OSLP T1 ON T0.[SlpCode] = T1.[SlpCode]
WHERE T0.[Action] = 'M' AND
T0.[Recontact] >= 'date' AND
T0.[Recontact] <= 'date2' AND
T1.[SlpName] = 'user name'
GROUP BY YEAR(T0.[Recontact]), MONTH(T0.[Recontact]), T1.[SlpName]
ORDER BY 1,2

Answer 1

If the year + month is totally missing from your data, you'll need to construct an empty row somewhere that can be shown in the place. You can create either a calendar table (one row per day) or a month table (one row per month). That can also be a "virtual" tally table constructed in a CTE or similar.
Once you have that, you can do something like this:
select
M.Year, M.Month, X.SlpName, isnull(X.CODES,0) as CODES
from
months M
outer apply (
SELECT
YEAR(T0.[Recontact]) as Year,
MONTH(T0.[Recontact]) AS Month,
T1.[SlpName],
COUNT(T0.[ClgCode]) AS CODES
FROM OCLG T0
INNER JOIN OSLP T1 ON T0.[SlpCode] = T1.[SlpCode]
WHERE T0.[Action] = 'M' AND
T0.[Recontact] >= 'date' AND
T0.[Recontact] <= 'date2' AND
T1.[SlpName] = 'user name'
GROUP BY YEAR(T0.[Recontact]), MONTH(T0.[Recontact]), T1.[SlpName]
) X on X.Year = M.Year and X.Month = M.Month
where M.MONTHDATE >= 'date' and M.MONTHDATE <= 'date2'
ORDER BY 1,2
This was with an imaginary month table that has year, month and monthdate columns, and the date is the first of the month -- you'll still have to check that the range you're fetching is correct.
I haven't tested this, but it should work.

Answer 2

Replace INNER JOIN with LEFT JOIN in your request to get NULL results.
SELECT YEAR(T0.[Recontact]) AS 'Év', MONTH(T0.[Recontact]) AS 'Hónap',
T1.[SlpName], COUNT(T0.[ClgCode]) AS 'Tárgyalások'
FROM OCLG T0
LEFT JOIN OSLP T1 ON (T0.[SlpCode] = T1.[SlpCode]
AND T1.[SlpName] = 'user name')
WHERE T0.[Action] = 'M'
AND T0.[Recontact] >= 'date'
AND T0.[Recontact] <= 'date2'
GROUP BY YEAR(T0.[Recontact]), MONTH(T0.[Recontact]), T1.[SlpName]
ORDER BY 1,2

Answer 3

Replace INNER JOIN with LEFT JOIN in your request to get NULL results.
to learn more visit :
http://academy.comingweek.com/sql-groupby-clause/