The document discusses referential integrity in relational databases. It explains that referential integrity requires primary keys and foreign keys to be consistent, so that foreign keys always match existing primary keys in linked tables. This prevents issues like orphaned records or adding records that reference nonexistent keys. Referential integrity guarantees consistency between linked tables and helps maintain data integrity.

2.  Referential integrity is a relational database
concept, which states that table relationships
must always be consistent. In other words, any
foreign key field must agree with the primary key
that is referenced by the foreign key.
 Primary key field changes must be applied to all
foreign keys, or not at all. The same restriction
also applies to foreign keys in that any updates
(but not necessarily deletions) must be
propagated to the primary parent key.

3.  Databases use tables to organize the
information they contain. They are similar to
spreadsheets, such as Excel, but vastly more
capable for advanced uses.
 Databases function with the use of primary
keys and foreign keys, which maintain the
relationship between the tables.

4.  The primary key of a database table is a
unique identifier assigned to each record.
Each table will have one or more columns
designated as the primary key. A Social
Security number can be a primary key for a
database listing of employees because each
Social Security number is unique.
 Some database software—such as Microsoft
Access—assigns the primary key
automatically, but the random key has no real
meaning.

5.  A foreign key is an identifier in a table that
matches the primary key of a different table.
 The foreign key creates the relationship with a
different table, and referential integrity refers to
the relationship between these tables.
 When one table has a foreign key to another
table, the concept of referential integrity states
that you may not add a record to the table that
contains the foreign key unless there is a
corresponding record in the linked table.

6.  Using a relational database
management system with referential integrity
offers several advantages:
 Prevents the entry of duplicate data
 Prevents one table from pointing to a
nonexistent field in another table.
 Guaranteed consistency between "partnered"
tables

7.  Prevents the deletion of a record that
contains a value referred to by a foreign key
in another table
 Prevents the addition of a record to a table
that contains a foreign key unless there is a
primary key in the linked table

8.  Another example of Referential Integrity
is Employee and Department relationship. If we
have dept_id as foreign key in Employee table
than by using referential integrity constraints
we can avoid creating Employee without
department or non existing department. In
short Referential Integrity makes primary key
foreign key relationship viable. Let's first
create Employee and Department table
with primary key, foreign key and referential
Integrity constraints.

10.  SQL statements in previous slide will create
both Department and Employee table. dept
_id is now foreign key in Employee table.
In this SQL, while creating foreign key we
have specified ON DELETE clause which
tells, what needs to done when a record
from parent table is
deleted. CASCADE referential action allows
to delete or update all matching rows from
child table, after deleting a record in
parent table. This way Referential Integrity
preserve data integrity of relationship.

12.  In the previous slide, When we inserted first
record in Department table it ran fine but
when we insert a record in Employee table
with dept_id = 2 which is not present in
Department i.e. parent table, failed to
Referential integrity or foreign key constraint
check.