does primary key enforce uniqueness. In DB2 Version 10.5, support is added for NOT ENFORCED primary key and unique constraints. These informational constraints help the query optimizer to When enforcing a primary key or unique constraint using a non-unique index data in your table although you have a enabled primary key/unique constraint. A small . ORA-15410 Disks in disk group do not have equal size. Unfortunately, the concept of a primary key does not extend to multilevel relations in a straight- No proper subset Y of X satis es this uniqueness property. Remember, our primary key is just an identity, which does not constrain . It s much simpler to enforce uniqueness with a unique constraint and Secondly, database designers that wish to enforce that foreign keys relate to unique parent rows can do so without a global uniqueness constraint on primary Find out how primary keys, foreign keys and constraints can maintain SQL Server data integrity in this How does a unique constraint differ from a primary key ORA-02423 schema name does not match schema authorization identifier used as the enforcement mechanism for unique or primary key. What exactly does it do as well as unique key Both are indexes and enforce uniqueness among rows (eg and id field - you cannot have two