- NOT
NULL constraints
NOT NULL constraints prevent null values from being entered into
a column. - Unique
constraints
Unique constraints ensure that the values in a set
of columns
are unique and not null for all rows in the table. The columns specified
in
a unique constraint must be defined as NOT NULL. The database manager
uses
a unique index to enforce the uniqueness of the key during changes to
the
columns of the unique constraint. - Primary
key constraints
You can use primary key and foreign key constraints to define
relationships
between tables. - (Table)
Check constraints
A check constraint (also referred to as a table check
constraint) is a database rule that specifies the values allowed in
one
or more columns of every row of a table. Specifying check constraints
is
done through a restricted form of a search condition. - Foreign
key (referential) constraints
Foreign key constraints (also known as referential constraints or referential
integrity
constraints) enable you to define required relationships between
and within tables. - Informational
constraints
An informational constraint is a constraint
attribute that
can be used by the SQL compiler to improve the access to data.
Informational
constraints are not enforced by the database manager, and are not used
for
additional verification of data; rather, they are used to improve query
performance.