C# :: Lookup Non-existent Rows In MySQL Table And Then Update Another Table

I have written an SQL statement that would:

1) Count the number of rows in the table booking that are open and where the booking.postcode is "MK",

2) Take a note of the plot (in booking.plot_id), and then update the table plot.jobs with the value count.

For example running the SQL query when booking table has the following rows:

Would see the following highlighted values in plot.jobs being updated to 1:

I managed to resolve it with this code so far (note I am using Connector/Net):

public void RefreshPlot(){
string query =
"SELECT Count(*) AS count, plot_id FROM booking WHERE postcode='MK' AND status='open' GROUP BY plot_id";
var cmd = new MySqlCommand(query, _connection);
var da = new MySqlDataAdapter(cmd);
var dtCounts = new DataTable();
da.Fill(dtCounts);

[code]....

Currently, my code only checks for existing rows in the booking table and updates the plot table. However if the row gets deleted in the booking, then the changes are not reflected in the plot table.

Example: If we delete the row with plot.id=1 and plot.plot_id=4, then booking.plot_id should go back to being 0, if it was initially 1. At the moment, it doesn't. How would I update my SQL statement to better reflect this? In a sense, it should check for "non-existent" rows, i.e. the impact that the row plot.plot_id=4 & plot.id=1 has on booking.plot_id when deleted?