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Learn more about the collection functions EXISTS, EACH, ALL, COLLECT FROM WHERE, COLLECT FROM NAMED WHERE, COLLECT, UNPACK, LIST, SIZE, UNIQUE, SUBSET OF, UNION, INTERSECTION, DIFFERENCE, SYMMETRIC_DIFFERENCE.
Overview
Function | Description |
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EXISTS | Determines |
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EXISTS
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whether an instance of a specified entity exists, optionally meeting certain criteria |
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Syntax
Code Block |
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EXISTS instances [ WHERE ( condition ) ] |
- instances - Instances to search.
- condition - boolean expression that represents the criterion the instance has to meet.
Return type
- boolean
Examples
Suppose the following data model.
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type | note |
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Determines whether all instances of a specified entity meet a certain criteria |
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Syntax
Code Block |
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EACH instances WHERE ( condition ) |
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Return type
- boolean
Examples
Suppose the following data model.
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ALL | Creates |
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ALL
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a collection of all instances of a specified |
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Syntax
Code Block |
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ALL entity |
entity |
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Return type
- collection of entity instances
Examples
Suppose the following data model. The Parent
and Child
entities both have Person
as base entity.
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Creates a collection of entity or attribute instances (meeting certain criteria |
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Syntax
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) |
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Return type
- collection of entity instances
- collection of attribute values
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Examples
Suppose the following model. Entity Teacher
has a multivalued relation with entity Child
via the relation Teacher.teaches_Children
.
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Version of |
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COLLECT FROM [WHERE] |
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for complex nested selections |
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Syntax
Code Block |
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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
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A collections contains no duplicates. Intermediary |
Examples
Suppose the following model with only instances of entity Person
. Person.hasChildren
is a relation from Person to Person.
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To retrieve all the parent names, we use this expression:
Code Block |
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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:
Code Block |
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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:
Code Block |
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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:
Code Block |
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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:
Code Block |
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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 ) |
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with |
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type | note |
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an alias. |
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UNPACK
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UNPACK | Extracts the value from a single valued collection or list. |
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Is the inverse of the LIST function. |
Syntax
Code Block |
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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.
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LIST
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LIST | Creates a list based on a value. |
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Is the inverse of the UNPACK |
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Syntax
Code Block |
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LIST ( attribute/collection ) |
- attribute - An attribute of any data type.
- collection/list - A collection or list of values.
Return type
- a multivalued list with entries of any type
Examples
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Suppose the following data model.
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function |
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This function determines the size of a collection.
Syntax
Code Block |
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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:
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Then:
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SIZE and COUNT are similar except for ?: SIZE ( ? ) results in 0, while COUNT ( ? ) results in ? |
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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 with the COLLECT
statement however, can contain duplicates. See the note on collections and duplicates for more info.
Syntax
Code Block |
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UNIQUE ( collection ) |
- collection - A collection of attribute or entity instances.
Examples
Suppose the following model.
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- As Jenny has an unknown sequence number, the result of the
COLLECT
expression is?
. TheLIST
function creates an empty list in case the parameter has?
value.
SIZE | Determines the size of a collection. |
UNIQUE | Filters duplicate items from a collection resulting from a subexpression in a larger expression. |
SUBSET OF | Returns |
TRUE if the items in a collection are all present in another |
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Syntax
Code Block |
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collection1 SUBSET OF collection2 |
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collection |
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Return type
- boolean
Diagram
Examples
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Values between single quotes are considered value list items. For backwards compatibility reasons, a comma separated sequence of value list items is treated as a collection. That's why there is no need to enclose the values between square brackets. In fact if you do add the square brackets you create a matrix rather than a list. |
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UNION | Adds two collections of the same base type to a new collection |
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Syntax
Code Block |
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UNION ( collection1 , collection2 ) |
- collection1 - First collection to be added to the new collection.
- collection2 - Second collection to be added to the new collection.
Return type
- collection
Diagram
Examples
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:
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Determines the intersection of two |
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collections. |
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Syntax
Code Block |
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INTERSECTION ( collection1 , collection2 ) |
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Return type
- collection
Diagram
Examples
Suppose the following model. Entity Teacher
has a multivalued relation with entity Child
via the relation Teacher.teaches_Children
.
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DIFFERENCE | Determines the difference between 2 collections. Returns |
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DIFFERENCE
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a collection containing all the items from collection1 that are not present in collection2. |
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Syntax
Code Block |
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DIFFERENCE ( collection1 , collection2 ) |
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Return type
- collection
Diagram
Examples
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SYMMETRIC DIFFERENCE
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SYMMETRIC_DIFFERENCE | Determines the symmetric difference between two collections. |
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Returns a collection with the elements of the provided collections which are in either one of the collections, but not in both |
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Syntax
Code Block |
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SYMMETRIC_DIFFERENCE ( collection1 , collection2 ) |
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Return type
- collection
Diagram
Examples
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A note on collections and duplicates
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An expression resulting in a collection does not contain duplicates. Please be aware however, that intermediary results of a COLLECT
statement can contain duplicates. You have to be aware of this when using the SIZE
or UNPACK
function.
This is best illustrated with the following examples.
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Now an example with the UNPACK function. We leave out the first instance from the previous example.
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Functions
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Only the intermediary results of a |
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