Constraints are categorized as follows.
Domain
integrity constraints
ü Not null
ü Check
Entity
integrity constraints
ü Unique
ü Primary key
Referential
integrity constraints
ü Foreign key
Constraints are always attached to a column
not a table.
We can add constraints in three ways.
ü Column level -- along with the column definition
ü Table level -- after the table definition
ü Alter level -- using alter command
While adding constraints you need not
specify the name but the type only, oracle will internally name the constraint.
If you want to give a name to the
constraint, you have to use the constraint clause.
