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This function determines whether an instance of a specified entity exists, optionally meeting certain criteria.
Syntax
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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.
| Person.name | Person.gender | Person.age |
|---|---|---|
| “Kim” | “f” | 23 |
| “Rick” | “m” | 35 |
| “Bob” | “m” | 42 |
| “John” | “m” | 19 |
| “Mary” | “f” | 33 |
EXISTS Person= TRUEEXISTS Person WHERE (Person.age < 18)= FALSEEXISTS Person WHERE (Person.gender = “m” AND Person.age > 35)= TRUE
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The return value of a boolean can be |
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EACH
This function determines whether all instances of a specified entity meet a certain criteria
Syntax
| Code Block |
|---|
EACH instances WHERE condition |
- instances - A collection of instances to search.
- condition - Boolean expression that represents the criterion the instance has to meet.
Return type
- boolean
Examples
Suppose the following data model.
| Person.name | Person.gender | Person.age |
|---|---|---|
| “Kim” | “f” | 23 |
| “Rick” | “m” | 35 |
| “Bob” | “m” | 42 |
| “John” | “m” | 19 |
| “Mary” | “f” | 33 |
EACH Person WHERE (Person.age < 18)= FALSEEACH Person WHERE (Person.age > 18)= TRUEEACH Person WHERE (Person.age < 20)= FALSEEACH Person WHERE (Person.gender = “m” OR Person.age > 35)= FALSEEACH Person WHERE (Person.gender = “m” OR Person.age > 20)= TRUE
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Use this function to create a collection of entity or attribute instances (meeting certain criteria).
Syntax
| Code Block |
|---|
COLLECT entity | attribute FROM collection [WHERE expression] |
- entity or attribute - Entity or attribute to collect.
- collection - A collection of entity instances.
- 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. 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” |
- COLLECT Child.name FROM ALL Child results in a collection of the values “Kim”, “Rick”, “Bob” and “Mary”
- COLLECT Child FROM Teacher[Teacher_2].teaches_Children results in a collection of the instances Child_1, Child_3 and Child_4
- COLLECT Child.hobbies FROM Teacher[Teacher_1].teaches_Children results in a collection of the values “Reading”, “Dancing”, “Tennis”, “Painting” and “Basketball”
- COLLECT Child.name FROM ALL Child WHERE (Child.hobbies = “Reading”) results in a collection of the values “Kim” and “Bob”
- COLLECT Child.hobbies FROM ALL Child WHERE (Child.name = “Mary”) results in a collection of the value “Football”
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COLLECT FROM NAMED [WHERE]
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