>>>Code column seems to identify the product, I think it can be used as PK.
>>
>>A sales invoice might have product codes more than once.
>>So code column can not be used as primary key.
>
>So, what is the key - the invoice_no, product code + date ?

I think no,
Please imagine what will be the primary key if table has data like this

vou_no--code---Product---qty---rate--Amount
---1-------1401----Apple------10----15----150
---1-------1402----mango-----20----15----300
---1-------1401----Apple------10----12----120
---1-------1401----Apple------12----16----180
---1-------1402----mango-----20----13----260

code 1401 was used 3 times due to different rates
code 1402 was used 2 times due to different rates

In this situation, is there any chance to create ID?