The simplest type of expression constraint contains a single concept optionally preceded by an expression constraint operator and/or membership function. Expression constraint operators (e.g. descendant of) traverse the hierarchical relationships in SNOMED CT to return the set of concepts that are directly or transitively connected to the focus concept. Membership functions return the set of concepts referenced by a particular reference set.
In this section we consider some of these simple examples.

Self

If no expression constraint operator or membership function is applied, the expression constraint is satisfied only by the specified concept. For example, the expression constraint below is satisfied only by the concept 404684003 |clinical finding| . 404684003 |clinical finding|
Please note that this expression constraint is equivalent to an expression that looks the same but is written in SNOMED CT Compositional Grammar.

Descendant of

A single 'less than' sign (i.e. "<") indicates that the expression constraint is satisfied by all descendants of the specified concept. The expression constraint below evaluates to the set of all subtypes (both direct children and transitive subtypes) of 404684003 |clinical finding| , using the brief syntax. <  404684003 |clinical finding|
Using the long syntax, the above expression constraint may be represented as: descendantOf  404684003 |clinical finding|
The descendantOf function is primarily used on concepts, which serve as the 'grouper' of a set of values (e.g. 404684003 |clinical finding (finding)| ,  272141005 |severities (qualifier value)| ,  258666001 |unit (qualifier value)| ).

Descendant or self of

Two consecutive 'less than' signs (i.e. "<<") indicates that the expression constraint is satisfied by all descendants of the specified concept plus the specified concept itself. The expression constraint below evaluates to the set of descendants of 73211009 |diabetes mellitus| , plus the concept 73211009 |diabetes mellitus| itself. <<  73211009 |diabetes mellitus|
Using the long syntax, the above expression constraint may be represented as: descendantOrSelfOf  73211009 |diabetes mellitus|
The descendantOrSelfOf function is primarily used for attribute values, which refer to a specific clinical value (e.g. 73211009 |diabetes mellitus| ,  73761001 |colonoscopy| , 385055001 |tablet dose form| ), but any specialization of this value is also acceptable.

Child of

An exclamation mark directly followed by a 'less than' sign (i.e. "<!") indicates that the expression constraint is satisfied by the set of immediate children of the specified concept. The children of a concept are those concepts that are the source of a non-redundant |is a| relationship whose target is the given concept. The expression constraint below, represented using the brief syntax, evaluates to the set of immediate children of the concept 404684003 |clinical finding| . <!  404684003 |clinical finding|
Using the long syntax, the above expression constraint may be represented as: childOf  404684003 |clinical finding|

Ancestor of

A single 'greater than' sign (i.e. ">") indicates that the expression constraint is satisfied by all ancestors of the specified concept. The expression constraint below, using the brief syntax, evaluates to the set of all supertypes (both direct parents and transitive supertypes) of 40541001 |acute pulmonary edema| :  

  >  40541001 |acute pulmonary edema|
Using the long syntax, the above expression constraint may be represented as: ancestorOf  40541001 |acute pulmonary edema|

Ancestor or self of

Two consecutive 'greater than' signs (i.e. ">>") indicates that the expression constraint is satisfied by all ancestors of the specified concept plus the specified concept itself. The expression constraint below evaluates to the set of ancestors of 40541001 |acute pulmonary edema| , plus the concept 40541001 |acute pulmonary edema| itself.

  >>  40541001 |acute pulmonary edema|
Using the long syntax, the above expression constraint may be represented as: ancestorOrSelfOf  40541001 |acute pulmonary edema|

Parent of

An exclamation mark directly followed by a 'greater than' sign (i.e. ">!") indicates that the expression constraint is satisfied by the set of immediate parents of the specified concept. The parents of a concept are those concepts that are the target of a non-redundant |is a| relationship whose source is the given concept. The expression constraint below, represented using the brief syntax, evaluates to the set of immediate parents of the concept 40541001 |acute pulmonaryedema| . >!  40541001 |acute pulmonary edema|
Using the long syntax, the above expression constraint may be represented as: parentOf  40541001 |acute pulmonaryedema|

Member of

The memberOf function evaluates to the set of concepts that are referenced by the given reference set (i.e. the set of referencedComponentIds). Please note that this function may be applied only to reference sets whose referenced components are concepts. The SNOMED CT Expression Constraint language does not support use of the memberOf function on reference sets whose referencedComponents are not concepts (i.e. descriptions, relationships or reference sets).
The memberOf function is represented in the brief syntax using a 'caret' character (i.e. "^") and must be immediately followed by a single concept id for a concept-based reference set. For example, the following expression constraint is satisfied by the set of concepts which are members of Example problem list concepts reference set: ^  700043003 |example problem list concepts reference set|
Using the long syntax the expression constraint is represented as: memberOf  700043003 |example problem list concepts reference set|
Please note that to represent the members of a reference set, which is itself composed of members of two or more other reference sets, an intensional definition should be created. For example, to refer to the members of the ^ AAAAAA  |allergen reference set| AAAAAA |allergen reference set| , which is defined as containing the members of the DDDDDD  |drugallergen reference set| and the members of the FFFFFF  |food allergen reference set| , then a row should be inserted into a query reference set with:
referencedComponentId =AAAAAA
query = ^ DDDDDD  |drug allergen reference set|  OR ^ FFFFFF  |foodallergen reference set|
(where AAAAAA, DDDDDD and FFFFFF are the concept identifiers of the respective reference sets).
The expression constraint " ^ AAAAAA  |allergenreference set| " will then be satisfied by both the concepts referenced by DDDDDD  |drugallergen reference set| and the concepts referenced by FFFFFF  |food allergen referenceset| .

Any

A single 'star' (i.e. "*") may be used in the place of a concept reference to represent any concept in the substrate. The expression constraint below evaluates to the set of all concepts in the given substrate.
*
Using the long syntax, the above expression constraint may also be represented as:
ANY
This wildcard character (or 'ANY' keyword) may be used anywhere within an expression constraint that a concept reference may be used. In many situations, the wildcard is equivalent to the following expression constraint: <<  138875005 |SNOMED CT concept|
However, some situations exist in which the concept 138875005 |SNOMED CT concept| is not included in the substrate, and therefore cannot be used to determine the full set of concepts available. In other cases, the single character wildcard may serve as a convenient shortcut for the longer expression constraint above.
Please note that the following three expression constraints evaluate to the same set of concepts:
*
<< *
>> *
The expression constraint below evaluates to all concepts in the substrate minus the root concept:
< *
And the expression constraint below evaluates to all non-leaf concepts in the substrate:
> *
Finally, the expression constraint below evaluates to all concepts that are referenced by any reference set in the substrate:
^ *