Code help: Update table with open items count from other table?

This is for an airport parking lot using SQL Server. All parkers prepay when they arrive for the duration of their stay (multiple days). Parking tickets are stored in a detail table with the PurchasedDate and ExpirationDate. I created a summary table that needs to include every calendar day and a count of the number of parking tickets valid for each day. I can get the count of vehicles with the following code, but am stuck on how to update my summary table.

SELECT P.Occupancy_Date, P.Parking_Lot, P.Peak_Occupancy, COUNT(*) AS Car_Count --(This is the value I want to use to update the Peak_Occupancy field)
FROM Parking_Peak_Occupancy P
INNER JOIN Parking_Transactions T ON CAST(T.PurchasedDate AS DATE)<=P.Occupancy_Date AND T.ExpirationDate>=P.Occupancy_Date
GROUP BY P.Occupancy_Date, P.Parking_Lot, P.Peak_Occupancy;

TIA!

Just a reminder that this forum is meant to help users with questions about TDP product usage, not about algorithms, code, etc.

But while you're here, sounds like you might need to employ aggregation via your SQL (e.g. SUM or COUNT tickets when grouped by day, for example). Hope that gives you some direction.