Access foreign key not updating
Developers can start with a focus on their application domain by designing the conceptual model and let the database creation flow from that process.
This chapter builds on that material to go into more detail on several areas of SQL implemented by My SQL.
A UNIQUE constraint defines a set of columns that uniquely identify rows in a table only if all the key values are not NULL.
If one or more key parts are NULL, duplicate keys are allowed.
A constraint operates with the privileges of the owner of the constraint.
See "Using SQL standard authorization" and "Privileges on views, triggers, and constraints" in the A primary key defines the set of columns that uniquely identifies rows in a table.