what is primary key,unique key, foreign key? can u teach me
in simple language?
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Answer Posted / nitin bisht
PRIMARY KEY: A tabLe can aontain only one PRIMARY KEY. It
Doesn't allow null values, it is used as Foreign Key In
Another Table(like Reference Key).
UNIQUE KEY: A Table may Contain one or more UNIQUE
constraints. Its aloow only one null value . Use this key We
maintain unique vales in the table.
FOREIGN KEY: A key used in one table to represent the value
of a primary key in a related table. While primary keys must
contain unique values, foreign keys may have duplicates. For
instance, if we use student ID as the primary key in a
Students table (each student has a unique ID), we could use
student ID as a foreign key in a Courses table: as each
student may do more than one course, the student ID field in
the Courses table (often shortened to Courses.student ID)
will hold duplicate values.
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