- Integrity constraints :
- Integrity constraints = constraints that every tuple in a relational database must satisfy.
- Types of constraints:
- Key constraint
- Entity integrity constraint
- Referential integrity constraint
- Key constraint:
- Every relation schema
must
have a
primary key
NOTE:
- A key may
consist of
all attributes
in the relation !!!
(Usually, a key is a subset of the attributes in a relation).
- Every relation schema
must
have a
primary key
- Entity integrity constraint:
-
The
attribute values of the
primary key
cannot have
NULL values
This constraint make perfect sense:
- The primary key is used to
identify a tuple
- NULL
value means
Not Applicablex
or
Not Available
- In either case, the primary key would be crippled (useless) in its ability to identify a tuple.
-
The
attribute values of the
primary key
cannot have
NULL values
- Referential integrity constraint:
-
Suppose relation schema R1
contains a
foreign key
(which is the primary key) of
the relation schema R2
(in other words: R1
"points to"
R2).
- Then the following constraint must hold:
- if a tuple t1 in R1 that references the tuple t2 in R2, then the tuple t2 must exist
Pictorially explanation of Referential integrity constraint:
(This also makes perfect sense because you must point to something that exists !!!)
-
Suppose relation schema R1
contains a
foreign key
(which is the primary key) of
the relation schema R2
(in other words: R1
"points to"
R2).