Answer Posted / kiran chandra sarkar

PRIMARY KEY: A table can contain 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. It’s allowing 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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1158

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