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The Expression Constraint Language is a formal syntax for representing SNOMED CT expression constraints. Expression constraints are computable rules used to define a bounded sets of clinical meanings represented by either precoordinated or postcoordinated expressions. Expression constraints can be used to restrict the valid values for a data element in an EHR, as the intensional definition of a concept-based reference set, as a machine processable query that identifies a set of matching expressions, or as a constraint that restricts the range of an attribute defined in the SNOMED CT concept model.

The Expression Constraint Language - Specification and Guide has full details on ECL, including use cases, requirements, logical model, syntax specification, examples, and implementation considerations. Appendix D - ECL Quick Reference is reproduced below as a starting point for familiarisation, showing a syntax overview and some examples of each of the key syntax features. Other useful appendices for finding out more about its use include Appendix A – Examples Of Valid Expressions and Appendix B – Examples Of Invalid Expressions.


This section provides a quick reference to the key syntax features of the Expression Constraint Language.

Syntax Overview

The following table summarises the key symbols used in the Expression Constraint Language's brief syntax, with the ECL version in which each symbol was introduced. For more information about the version history of ECL, please refer to the 'History' section in 1. Introduction.

Symbol

Name

VersionNotes
|Pipe1.0Used on either side of a concept's term for human readability
*Any1.0Retrieves all concepts in the substrate
^Member of1.0Retrieves the referencedComponentId of all (active) members of a reference set (or set of reference sets)
^ [ A, B]Member of (with field selection)2.0Retrieves the values of fields A and B of all (active) members of a reference set (or set of reference sets) that match the included Member filters (if applicable)

<

Descendant of

1.0Retrieves all descendants (subtypes) of the specified concept excluding the concept itself

<<

Descendant or self of

1.0Retrieves all descendants (subtypes) of the specified concept including the concept itself

<!

Child of

1.1Retrieves all children (immediate subtypes) of the specified concept excluding the concept itself

<<!

Child or self of

1.4Retrieves all children (immediate subtypes) of the specified concept including the concept itself
>Ancestor of1.0Retrieves all ancestors (supertypes) of the specified concept excluding the concept itself

>>

Ancestor or self of

1.0Retrieves all ancestors (supertypes) of the specified concept including the concept itself

>!

Parent of

1.1Retrieves all parents (immediate supertypes) of the specified concept excluding the concept itself

>>!

Parent or self of

1.4Retrieves all parents (immediate supertypes) of the specified concept including the concept itself

AND

Conjunction

1.0Retrieves the intersection of the results of each sub-expressions

OR

Disjunction

1.0Retrieves the union of the results of each sub-expressions

MINUS

Exclusion

1.0Retrieves the members of the first expression and excludes the members returned by the second expression
:Refinement1.0Used before one or more attribute-value pairs to refine the set of concepts retrieved

[1..3]

Cardinality

1.0Used to indicate the minimum and maximum number of occurrences of attributes or relationship groups

R

Reverse flag

1.0Retrieves the set of attribute values (i.e. destination concepts) of a specified attribute for a specified set of concepts

.

Dot notation

1.2Retrieves the set of attribute values (i.e. destination concepts) of a specified attribute for a specified set of concepts
/*  */Comment1.1Allows comments to be added within the text of an expression constraint
{{  }}Description filter1.5Filters the result set, by matching only on concepts which have a description with a matching term, language, type, dialect and/or acceptability
{{ D }}Description filter1.6Filters the result set, by matching only on concepts which have a description with a matching term, language, type, dialect and/or acceptability
{{ C }}Concept filter1.6Filters the result set based on the definition status, module, effectiveTime and active status of each concept
{{ M }}Member filter2.0Filters the result set based on the value of specific fields in a reference set.
{{+ HISTORY}}History supplement2.0Supplements the results with relevant inactive concepts

Examples

The following table provides some examples of each of the key syntax features of the Expression Constraint Language.

Notes

  1. In the table above:
    • 'id' represents a single SNOMED CT concept identifier,
    • 'term' represents a term associated with the concept identified by 'id',
    • 'x', 'y' and 'v' each represent either a single concept or a set of concepts defined using an expression constraint,
    • 'z' represents either a single concept or a set of concepts that are a subtype of 900000000000455006 | Reference set|
    • 'a' and 'b' each represent either a single concept or a set of concepts that are a subtype of 410662002 | Concept model attribute| , and
    • 'min' and 'max' are two numeric values that represent the minimum and maximum cardinality allowed.
  2. The default substrate, to which expression constraints are applied, includes all concepts, active relationships, active descriptions and active reference set members of a chosen SNOMED CT versioned edition.
Simple expression constraints
SyntaxEvaluation NotesExampleExample Expansion Concepts
id | term |
Only the concept with the identifier 'id'
*
All concepts in the given substrate

*

Any concept in the given substrate
^ z
The set of concepts which are members of the reference sets in z
< x
The set of all descendants (both direct and indirect) of x

73211009 |Diabetes mellitus|

< 73211009 |Diabetes mellitus)|

<< x
The set of all descendants (both direct and indirect) of x, plus x itself
<! x
The set of all immediate children of x
<<! x
The set of all immediate children of x, plus x itself
> x
The set of all ancestors (both direct and indirect) of x
>> x
The set of all ancestors (both direct and indirect) of x, plus x itself
>! x
The set of all immediate parents of x
>>! x
The set of all immediate parents of x, plus x itself
Conjunction, Disjunction and Exclusion
SyntaxEvaluation NotesExampleExample Expansion Concepts
x AND y
The set of concepts that are both in x and in y (i.e. the intersection of x and y)
x OR y
The set of concepts that are either in x or in y (i.e. the union of x and y)
x MINUS y
The set of concepts that are in x but are not in y (i.e. x excluding concepts in y)
Refinement
SyntaxEvaluation NotesExampleExample Expansion Concepts

x : a = y

The set of concepts in x, which have a necessary relationship with an attribute in a and a value in y
x : a = y, b = v
The set of concepts in x, which have both a necessary relationship with an attribute in a and a value in y, and also have a necessary relationship (either the same one or a different one) with an attribute in b and a value in v
x : { a = y, b = v }The set of concepts in x, which have a role group that contains both a necessary relationship with an attribute in a and a value in y, and also have a necessary relationship (either the same one or a different one) with an attribute in b and a value in v
Cardinality
SyntaxEvaluation NotesExampleExample Expansion Concepts
x : [min .. max] a = y
The set of concepts in x, which have between min and max necessary relationships with an attribute in a and a value in y
x : [min .. max] { a = y }
The set of concepts in x, which have between min and max role groups that contain a necessary relationship with an attribute in a and a value in y


Reversed Attributes
SyntaxEvaluation NotesExampleExample Expansion Concepts
y : R a = x

The set of concepts in y, which are the destination (ie attribute value) of a necessary relationship on a source concept in x with an attribute in a

x . a
The set of attribute values (ie destination concepts) of all necessary relationships on a source concept in x with an attribute in a