Data integrity 

• Integrity rules: the database system provides a wide range of declarative integrity rules, thus reducing the programming requirements for applications. Integrity rules that refer to a table can also be specified (see constraint name).
• Key: a key comprising one or more columns can be defined for each table. The database system ensures that each key is unique. A key can also consist of columns of different data types (see key definition).
• UNIQUE definition: the uniqueness of the values in other columns and column combinations can also be ensured by using other mechanisms (see UNIQUE definition for "alternate keys").
• NOT NULL: by specifying NOT NULL, you can ensure that the NULL value is not accepted in individual columns.
• DEFAULT definition: you can define default values for each column (see DEFAULT specification).
• Referential integrity conditions: by specifying referential integrity conditions, you can declare deletion and existence dependencies between the rows in two tables (see name of a referential constraint).
• Database procedures and triggers: complex integrity rules that require access to further tables can be formulated with database procedures or triggers.