Primary key or natural key – which has mapping to the someone in the real-world scenario that is not in database. Uniquely identifies a record in a table. Cannot be NULL.
For eg : ssn _number.
Foreign Key - A field in one table that refers to the primary key in another.
Surrogate key is again a primary key, but it has not mapped to anything in the real world, which is key only in database. “Numeric number “.
Composite key: - when we have multiple columns need to identify each row of records, then the primary key is a composite key.
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