将此触发器添加到您的ExpenseTable

Add this trigger to your ExpenseTable

CREATE TRIGGER ExpenseSum AFTER INSERT ON ExpenseTable FOR EACH ROW
BEGIN
    UPDATE ProjectsTable P
    SET ExpenseTotal = 
    (SELECT SUM(ExpenseAmount) from ExpenseTable
    where ExpenseTable.ProjectID= P.ProjectID)
    where P.ProjectID = New.ProjectID;
END

不要忘记添加触发器 After Update 和 After Delete 来更新ExpenseTotal

Don't forget to add trigger After Update and After Delete to update ExpenseTotal