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.