如何联接多个表，而不忽略不匹配的值

Question

This small example is based on a question that I have run into countless times, however I failed finding the best answer.

I would like to create a report on Incidents logged per type, per month. Written the following query.

SELECT
    d.MonthPeriod
    ,i.[Type]
    ,COUNT(*) AS [Count of Calls]
FROM
    [dbo].[FactIncident] as i
        LEFT JOIN
    [dbo].[DimDate] as d on i.DateLoggedKey = d.DateKey
GROUP BY
    d.[MonthPeriod],
    i.[Type]

This results in the following:

Although correct, I would like to visualize earlier months with 0 logged calls. DimDate contains the following.

What is the best and/or most efficient way of showing the count of calls per month, per type, for all months. Even if the count is 0?

Thought of using Cross Apply, however the resultant query gets huge quickly. Only think of a dataset requiring the count of calls per customer, per category, per month over the last 3 years..

Any ideas?

Answer 1

Do the left join starting with the calendar table, so you keep all the months:

SELECT d.MonthPeriod, i.[Type], COUNT(i.type) AS [Count of Calls]
FROM [dbo].[DimDate] d LEFT JOIN
     [dbo].[FactIncident] i
     ON i.DateLoggedKey = d.DateKey
GROUP BY d.[MonthPeriod], i.[Type];

This will, of course, return the type as NULL for the months with no data.

If you want all types present, then use CROSS JOIN on the types. This example gets the data from the fact table, but you might have another reference table containing each type:

SELECT d.MonthPeriod, t.[Type], COUNT(i.type) AS [Count of Calls]
FROM [dbo].[DimDate] d CROSS JOIN
     (select distinct type from factincident) t LEFT JOIN
     [dbo].[FactIncident] i
     ON i.DateLoggedKey = d.DateKey and i.type = t.type
GROUP BY d.[MonthPeriod], t.[Type];