# Storing

Storing SNOMED CT Expression Constraints in an expression constraint library may be done for a variety of purposes, including:

* To enable expression constraints to be re-executed (without re-authoring) after updates are made to the SNOMED CT substrate or the expression constraint itself;
* To provide a library of terminology binding constraints against which record instances will be validated;
* To provide a library of concept model constraints against which terminology artefacts (e.g. extensions, expressions) will be validated;
* To provide a library of predefined queries that may be shared by multiple users;
* To provide a library of terminology binding constraints that may be shared within a standards community.

A library of SNOMED CT Expression Constraints may be implemented using a number of techniques, including:

* Creating a Query specification reference set that records the expression constraint as the 'query';
* Creating a customized RF2 reference set with one or more new attributes that allow the expression constraint string and relevant metadata to be recorded;
* Creating a table in a relational database to store the SNOMED CT Expression Constraint and associated metadata;
* Creating a text file with a consistent structural format to store the SNOMED CT Expression Constraint and associated metadata;

In many cases it is useful to assign a unique identifier to each expression constraint in the library, so that they can be indexed and referenced for faster retrieval.

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