>>>>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?

Sounds like vou_no, code and Rate are the PK. Alternatively we can just add auto-increment identity column to be a PK and the combination of vou_no, code, Rate will be unique index.