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  • By design Foreign Key must refer to some key(Primary) which is not NULL, but during development phase when we need to have multiple data first inserted into child table, which we don't know whom it will refer to (the parent table). That is why we have NULL value allowed. In production having NULL will be a design flow, that can be roughly said. Commented Sep 17, 2015 at 10:12
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    @vimalkrishna Not neccessarily - the primary key cannot contain null, but the foreign key can be null if the child doesn't neccessarily have a parent element. For example if the parent elements are warehouses, and the child elements are products, a NULL value can mean that the product is not currently stored in a warehouse. This makes much more sense than creating a virtual "no warehouse" warehouse to satisfy a foreign key constraint that doesn't allow nulls. Commented Nov 24, 2020 at 12:23