Note that we do not have separate values for RCC buildings with less or equal than 4 floors, and with more than 3 floors. This is because these percentages were already calculated earlier.
First we need to calculate the number of buildings of the different building types (AD, BM, BC and RCC). We should do this in the table mapping_units. However, since we will calculate quite some columns, we will make a new table linked to the map Mapping_units, and call it Buildings_per_unit.