Can a database table exist without a primary key?

im doing a project for school at the moment.. one of the table in my database is for sales, (see attached photo) i was told by my lecturer to separate the table because of the fact that a customer can purchase more than 1 product and that may not be reflected in the table, the suggested table is Sales details, with... show more im doing a project for school at the moment.. one of the table in my database is for sales, (see attached photo) i was told by my lecturer to separate the table because of the fact that a customer can purchase more than 1 product and that may not be reflected in the table, the suggested table is Sales details, with columns receipt number, product_id, qty and cost. my question is would receipt number be the primary key in the sales details table or do i leave it unspecified cause a receipt number can contain more than one entry
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