The concept of relationship strength is based on how the primary key of a related entity is defined.
A primary key is a specific selection from a minimal set of attributes that uniquely specify a tuple within a relation. Informally, a primary key is an "attribute that identifies a record", most simply a single attribute (a unique ID).
A primary key is a column (or set of columns) in a table that uniquely identifies a row of data in the table. For example, in the following table, CustomerNo is the primary key, his ID number assigned to various customers.
A primary key (also called primary keyword) is a key in a relational database that is unique for each record.
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