Adding columns to a database afterwards

[Bizz]

Hello... I have a problem; I have created a coding with PHP that calculates the purchase price, sale price, stock quantity, etc. of materials. I have entered the data, but now I want to add a new column to the database without it being empty. I want to create a new column where I can calculate the stock quantity multiplied by the purchase price and add it. This is to find out the total value of goods in stock. Are there any friends who could help?