Explain when the primary key fields of two data tables need to be the same. ?

Explain when the primary key fields of two data tables need to be the same. ?
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Due to the following condition of the relation, the primary key fields of the data tables of the two databases need to be the same to create the relation. 1. The common field must be the primary key field and the foreign key field. 2. Common field name , data type , field size , format etc. should be the same and two tables should be open at the same time.
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