From the example in Entity and Entity Type, clearly a customer buys products. That is, an entity belonging to the entity type CUSTOMER can buy an entity belonging to the entity type PRODUCT. The reason for associating CUSTOMERs and PRODUCTs (that is, buys) is named a relationship. A relationship is a reason of relevance to the enterprise for associating entities (that is, occurrences) of one or two entity types.
A pairing is an occurrence of an association between two entities for the reason that is described by a relationship. So, if James Molt (a CUSTOMER) buys a ball bearing (a PRODUCT), the entities James Molt, and Ball bearings are said to be paired based on the relationship buys. In other words, a pairing is an occurrence of a relationship between two entities, while a relationship is a type of association between one or two entity types.
In this example, the entities are of two entity types, but pairings are also found between entities of the same type.
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