I don't understand the diagram in Relationship. Customer to Sales: It's one to many. So the symbol used near Sales table is >|. But From Sales to Customer, it is many to one but why are we writing the symbol as || ???

he supposes that the sales can't take more than one customer. it's an example, not more. you are right, it can be many to one (more than customer buy one sale) .like Customer to Sales > | means | minimum one .... > maxium many || means | minimum one | and max is one too (to achieve one to many relation)

Hi Moe and Mahmoud! Thanks for reaching out. @Mahmoud: Thank you very much for providing this clarification! Please let us know if you need further assistance. Thank you. Best, Martin