Structured Query Language (SQL) - Cultural View of Technology

Check Constraint 20
NOT NULL Constraint
A NOT NULL constraint is functionally equivalent to the following check constraint with an IS NOT NULL
predicate:
CHECK (column IS NOT NULL)
Some relational database management systems are able to optimize performance when the NOT NULL constraint
syntax is used as opposed to the CHECK constraint syntax given above. [1]
Common Restrictions
Most database management systems restrict check constraints to a single row, with access to constants and
deterministic functions, but not to data in other tables, or to data invisible to the current transaction because of
transaction isolation.
Such constraints are not truly table check constraints but rather row check constraints. Because these constraints are
generally only verified when a row is directly updated (for performance reasons,) and often implemented as implied
INSERT or UPDATE triggers, integrity constraints could be violated by indirect action were it not for these
limitations. Future, otherwise-valid modifications to these records would then be prevented by the CHECK
constraint. Some examples of dangerous constraints include:
• CHECK ((select count(*) from invoices where invoices.customerId = customerId) < 1000)
• CHECK (dateInserted = CURRENT_DATE)
• CHECK (countItems = RAND())
User-defined triggers can be used to work around these restrictions. Although similar in implementation, it is
semantically clear that triggers will only be fired when the table is directly modified, and that it is the designer's
responsibility to handle indirect, important changes in other tables; constraints on the other hand are intended to be
"true at all times" regardless of the user's actions or the designer's lack of foresight.
References
[1] PostgreSQL 8.3devel Documentation, Chapter 5. Data Definition, Section 5.3.2. Not-Null Constraints, Website: http://developer.postgresql.
org/pgdocs/postgres/ddl-constraints.html, Accessed on May 5, 2007