Editing Chemical database (section)

== Registration systems ==
Databases systems for maintaining unique records on [[chemical compound]]s are termed as Registration systems. These are often used for chemical indexing, [[patent]] systems and industrial databases.

Registration systems usually enforce uniqueness of the chemical represented in the database through the use of unique representations. By applying rules of precedence for the generation of stringified notations, one can obtain unique/'[[canonical form|canonical]]' string representations such as 'canonical [[SMILES]]'. Some registration systems such as the CAS system make use of algorithms to generate unique [[hash code]]s to achieve the same objective.

A key difference between a registration system and a simple chemical database is the ability to accurately represent that which is known, unknown, and partially known.  For example, a chemical database might store a molecule with [[stereochemistry]] unspecified, whereas a chemical registry system requires the registrar to specify whether the stereo configuration is unknown, a specific (known) mixture, or [[racemic]].  Each of these would be considered a different record in a chemical registry system.

Registration systems also preprocess molecules to avoid considering trivial differences such as differences in [[halogen]] ions in chemicals.

An example is the [[Chemical Abstracts Service]] (CAS) registration system. See also [[CAS registry number]].