What are constraints?It is the rules that prevent the invalid entry into the table. They are stored in the data dictionary. They can be defined either at column level or table level.
Following are the constraints available in oracle.
Not Null - Specifies that column can’t contain a null value.
Unique - Enforce unique value for all rows in the table.
Primary key - Uniquely identifies each row of the table.
Foreign key - Enforces a foreign key relationship between the columns of the referenced table.
Check - specifies condition that must be true.