Really answer are not simple by looking at your question you ask

1- No I don't have that data. Only sale report

2- If its related to sale so by seeing the sale report of every month I can check what brand or sku sale fast base of no of unit sold. Otherwise I don't clue.

3- Yes have warehouse

4- Its vary on product to product.

5- Yes they give discount on off seasonal product

Actually I'm new to excel don't know too much and my work is related to inventory. So I was looking what type of analysis i can be easy for beginner. I saw you reply earlier but I was keep reading response to understand the factor which you mention.