Key fields are the subset of fields within a table for which data MUST be entered and validated before a new record can be added to the table. Failure to correctly enter data in all the key fields will prevent a new record from being added to the table.
In the below example, c1 and c2 are key fields, but c3 is not:
CREATE TABLE t1( c1 NUMBER PRIMARY KEY, c2 VARCHAR2(30) NOT NULL, c3 VARCHAR2(10) );
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