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Use this version of the COLLECT FROM [WHERE] function for complex nested selections in which it is necessary to use an alias.
Syntax
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COLLECT entity | attribute FROM collection NAMED alias [WHERE expression] |
- entity or attribute - Entity or attribute to collect. This should contain the alias, e.g. alias.Name.
- collection - A collection of entity instances.
- alias - A name for the collection.
- expression - Boolean expression that represents the criterion the instance has to meet.
Return type
- collection of entity instances
- collection of attribute values
Examples
Suppose the following model with only instances of entity Person. Person.hasChildren is a relation from Person to Person.
Person instance | Person.Name | Person.Age | Person.hasChildren |
---|---|---|---|
Person_1 | “Kim” | 16 | |
Person_2 | “Rick” | 38 | Person_3, Person_5 |
Person_3 | “Bob” | 8 | |
Person_4 | “Julia” | 42 | Person_1 |
Person_5 | “Sam” | 3 | |
Person_6 | “Joan” | 34 | Person_3, Person_5 |
To retrieve all the parent names, we use this expression:
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COLLECT Parent.name FROM COLLECT Person FROM ALL Person WHERE (Person.hasChildren != ?) NAMED Parent |
This expression results in “Rick”, “Julia” and “Joan”.
To retrieve all children names, we use this expression:
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COLLECT Child FROM ( COLLECT Person.hasChildren FROM ALL Person WHERE (Person.hasChildren != ?)) NAMED Child |
This expression results in a collection of three instances of Person (i.e Person_1, Person_3 and Person_5, with names Kim, Bob and Sam).
To retrieve all children younger than 15, we use this expression:
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COLLECT Child FROM ( COLLECT Person.hasChildren FROM ALL Person WHERE (Person.hasChildren != ?)) NAMED Child WHERE (Child.Age < 15) |
This expression results in a collection of two instances of Person (i.e Person_3 and Person_5, with names Bob and Sam).
To retrieve the children’s names with a parent older than 40 years:
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COLLECT Child.Name FROM ( COLLECT Person.hasChildren FROM ALL Person WHERE (Person.hasChildren != ? AND Person.Age > 40)) NAMED Child |
This expression results in a collection with only the name “Kim”.
You can make it as complex as you like, for example:
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COLLECT Child.Name FROM ( COLLECT Parent.hasChildren FROM ( COLLECT Person FROM ALL Person WHERE (Person.hasChildren != ?)) NAMED Parent WHERE (Parent.Age > 40 ) ) NAMED Child WHERE (Child.Age < 18) |
This expression results also in a collection with only the name “Kim”.
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In some occasions, a local variable could be a good alternative for using an alias. E.g. if you want a collection of all persons except the person that is currently active, this would be simple and transparent:
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UNPACK
Use this function to extract the value from a single valued collection or list.
Syntax
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UNPACK collection/list |
- collection/list - A collection or list of one entity or attribute instance.
Return type
- entity instance
- attribute value of any type
Examples
Suppose the following data model.
Person.name | Person.SequenceNumber |
---|---|
“Bob” | 654 |
“Jane” | 523 |
“Mary” | 667 |
“Rick” | 500 |
“Ron” | 490 |
“Jenny” | 765 |
- UNPACK(COLLECT Person.name FROM ALL Person WHERE (Person.SequenceNumber = MIN(COLLECT Person.SequenceNumber FROM ALL Person))) results in “Ron”
- In case a second entry "Ron","490" exists, the expression "UNPACK(COLLECT Person.name FROM ALL Person WHERE (Person.SequenceNumber = MIN(COLLECT Person.SequenceNumber FROM ALL Person)))" will fail, because the UNPACK cannot resolve a list with two elements. To solve this the UNIQUE function has to be used: "UNPACK(UNIQUE(COLLECT Person.name FROM ALL Person WHERE (Person.SequenceNumber = MIN(COLLECT Person.SequenceNumber FROM ALL Person))))", which will result in "Ron".
- UNPACK(COLLECT Person.name FROM ALL Person WHERE (Person.SequenceNumber = MAX(COLLECT Person.SequenceNumber FROM ALL Person))) results in “Jenny”
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This function determines the size of a collection.
Syntax
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SIZE ( collection ) |
- collection - A collection of attribute or entity instances. This can be an expression or a relation attribute for instance.
Return type
- integer
Example
Suppose you have a Parent
and a Child
entity, where Parent
has a multivalued relation with Child
via the relation Parent.has_Children
. With this model the following instances are created:
Parent instance | Child instance | Child.name | Child.hobbies |
---|---|---|---|
Parent_1 | Child_1 | “Kim” | “Reading”, “Dancing” |
Parent_1 | Child_2 | “Rick” | “Tennis”, “Dancing” |
Parent_1 | Child_3 | “Bob” | “Painting”, “Basketball”, “Reading” |
Parent_2 | Child_4 | “Mary” | “Football” |
Then:
- if
Parent_1
is active,SIZE ( Parent.has_Children )
= 3 - if
Parent_2
is active,SIZE ( Parent.has_Children )
= 1 - without an active
Parent
instance,SIZE ( Parent.has_Children )
results in an error - if
Child_1
is active,SIZE ( Child.hobbies )
= 2 - if
Child_3
is active,SIZE ( Child.hobbies )
= 3 - if
Child_4
is active,SIZE ( Child.hobbies )
= 1 SIZE ( ? )
= 0
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COUNT and SIZE are similar except for UNKNOWN : COUNT ( ? ) = UNKNOWN |
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UNIQUE
The UNIQUE
function filters duplicate items from a collection. An expression resulting in a collection, never contains duplicate values. A subexpression however, can contain duplicates.
Syntax
Code Block |
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UNIQUE ( collection ) |
- collection - A collection of attribute or entity instances.
Examples
Suppose the following model.
Person instance | Person.name |
---|---|
Person_1 | “Kim” |
Person_2 | “Rick” |
Person_3 | “Bob” |
Person_4 | “Rick” |
COLLECT Person.name FROM ALL Person
= "Kim”, “Rick”, “Bob” (a result never contains duplicate values)SIZE ( COLLECT Person.name FROM ALL Person )
= 4 (a subexpression can contain duplicate values)SIZE ( UNIQUE ( COLLECT Person.name FROM ALL Person ) )
= 3 (the collection holds three unique values)
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This function determines the intersection of two collections. It returns a collection containing the items that are present in both specified collections.
Syntax
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INTERSECTION ( collection1 , collection2 ) |
- collection1, collection2 - Collections to be intersected. These collections must be of the same base type.
Return type
- collection
Examples
Suppose the following model. Entity Teacher
has a multivalued relation with entity Child
via the relation Teacher.teaches_Children
.
Teacher instance | Child instance | Child.name | Child.hobbies |
---|---|---|---|
Teacher_1 | Child_1 | “Kim” | “Reading”, “Dancing” |
Teacher_1 | Child_2 | “Rick” | “Tennis”, “Dancing” |
Teacher_1 | Child_3 | “Bob” | “Painting”, “Basketball”, “Reading” |
Teacher_2 | Child_1 | “Kim” | “Reading”, “Dancing” |
Teacher_2 | Child_3 | “Bob” | “Painting”, “Basketball”, “Reading” |
Teacher_2 | Child_4 | “Mary” | “Football” |
INTERSECTION ( Teacher[Teacher_1].teaches_Children , Teacher[Teacher_2].teaches_Children )
results in a collection ofChild
instancesChild_1
andChild_3
INTERSECTION ( Teacher[Teacher_1].teaches_Children.name , Teacher[Teacher_2].teaches_Children.name )
= “Kim”, “Bob”INTERSECTION ( Child[Child_1].hobbies , Child[Child_3].hobbies )
= “Reading”INTERSECTION ( Child[Child_2].hobbies , Child[Child_3].hobbies )
results in an empty list
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DIFFERENCE
This function returns a collection containing all the items from collection1 that are not present in collection 2.
Syntax
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DIFFERENCE ( collection1 , collection2 ) |
- collection1, collection2 - Collections to be intersected. These collections must be of the same base type.
Return type
- collection
Examples
DIFFERENCE ( [ "a", "b", "c"] , [ "c", "d", "e" ] )
= [ "a" , "b" ]DIFFERENCE ( [ "nv" , "bv" ] , [ "NV" ] )
= [ "bv" ]DIFFERENCE ( 1 , 1 )
results in an empty list
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SYMMETRIC_DIFFERENCE
This function determines the symmetric difference between two collections. It returns a collection with the elements of the provided collections which are in either one of the collections, but not in both.
Syntax
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SYMMETRIC_DIFFERENCE ( collection1 , collection2 ) |
- collection1, collection2 - Collections to be intersected. These collections must be of the same base type.
Return type
- collection
Examples
SYMMETRIC_DIFFERENCE ( [ "a", "b", "c"] , [ "c", "d", "e" ] )
= [ "a", "b", "d", "e" ]SYMMETRIC_DIFFERENCE ( [ "nv" , "bv" ] , [ "NV" ] )
= [ "bv" ]SYMMETRIC_DIFFERENCE ( 1 , 1 )
results in an empty list
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Collections vs. lists
UNIQUE, SUBSET, UNION and INTERSECTION return a collection of data, i.e. double entries are removed. However, COLLECT statements, + and - return a list of data. You have to be aware of this when using the SIZE or UNPACK function.
Example
Statuses = ["a","b","c","d","a","b","c","d"].
Result
The expression "SIZE ( COLLECT Status FROM {Statuses} )" will result in 8. Note that duplicates are not removed.
The expression "SIZE ( UNIQUE ( COLLECT Status FROM {Statuses} ) )" will result in 4, as the unique filters the duplicates.
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When the expression is completely evaluated, all duplicates are filtered as well. Using COLLECT Status FROM {Statuses} as a default expression on an attribute, will result in 4 elements. |
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