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A SNOMED CT Expression Constraint contains either a single focus concept, or a series of focus concepts joined by either conjunction, disjunction or exclusion. Each focus concept in an Expression Constraint is either a concept reference or a wildcard, and is normally preceded by either a constraint operator or a memberOf function. An Expression Constraint may also contain a refinement, which consists of grouped or ungrouped attributes (or both). Each attribute consists of the attribute name (optionally preceded by a cardinality, reverse flag and/or attribute operator) together with the value of the attribute. The attribute name is either a concept reference or a wild card. The attribute value is either an expression constraint or a concrete value (i.e. string, integer, decimal or boolean). Conjunction or disjunction can be applied at a variety of levels, including between expression constraints, refinements, attribute groups, and attributes. An expression constraint can also be followed by a dot and attribute name pair. One or more description filters may be applied to an expression constraint, which can include description identifier, module, effective time, active status, term, language, type, dialect and acceptability criteria. Similarly, one or more concept filters may be applied to an expression constraint, which can include definition status, module, effective time and active status criteria. Member filters may be applied to results of the memberOf function, and may include module, effective time, active status and specific refset field criteria. Finally, history supplements may be applied, which include an ECL query to specify the set of historical association reference sets to be used.

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Figure 1: Abstract Model of a SNOMED CT Expression Constraint

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