Constraints:
Not Null A value must be supplied for this column, but values do not have to be unique.
Unique Key Every value in this column must be unique, but null values are allowed.
Primary Key Every value in the column must be unique and cannot be null.
Foreign Key Every value in the column must match a value in another column in this table or some other table; otherwise, the value is null.
Check The value entered in the table must match one of the specified values for this column.
Constraint information is stored in the DBA_CONSTRAINTS and DBA_CONS_COLUMNS data dictionary views.
In addition to defining relationships between tables, you can also use foreign keys to define relationships between two columns within the same table. These types of constraints are referred to as self-referencing foreign keys.
Use ON DELETE CASCADE to remove data with child record.
