Module Documentation: eml-constraint
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This schema defines the integrity constraints between entities (e.g., data tables) as would be maintained in a relational database. These constraints include primary key constraints, foreign key constraints, unique key constraints, check constraints, and not null constraints, among potential others.

Element Definitions:

eml-constraint
  Tooltip: Relational integrity constraint descriptors  
  Summary: Describes the relational integrity constraints of a relational database.  
  Description: Describes the relational integrity constraints of a relational database. This includes primary keys, foreign keys, unique keys, etc.  
  Example:  
identifier
  Tooltip: Unique identifier  
  Summary: The unique identifier of this metadata file or object.  
  Description: The identifier field provides a unique identifier for this metadata documentation. It will most likely be part of a sequence of numbers or letters that are meaningful in a larger context, such as a metadata catalog. That larger system can be identified in the "system" attribute. Multiple identifiers can be listed corresponding to different catalog systems.  
  Example: <identifier system="metacat">nceas.3.2</identifier>  
constraint
  Tooltip: Integrity constraint  
  Summary: Information about the integrity constraint being defined.  
  Description: Complete information needed to reconstruct this integrity constraint for enforcement in a system such as a relational database management system.  
  Example:  
constraintType
  Tooltip: type of constraint  
  Summary: the type of constraint being defined  
  Description: the type of constraint being defined. This is what kind of constraint this is.  
  Example: PrimaryKey, ForeignKey, UniqueKey  
constraintName
  Tooltip: constraint name  
  Summary: name of the field that this constraint is tied to  
  Description: name of the field that this constraint is tied to. This could also be the name of an independent constraint such as a trigger or an autonumber generator.  
  Example: LocationID, TABLE1_NUMBER_GEN1  
constraintDescription
  Tooltip: constraint description  
  Summary: description of the constraint  
  Description: description of the constraint. Any additional information that may be needed to fully understand the constraint.  
  Example:  
entityId
  Tooltip: table to which this constraint is tied  
  Summary: table to which this constraint is tied  
  Description: table to which this constraint is tied. In the instance of a key, it is the name of the table in which it is defined.  
  Example: site_properties  
key
  Tooltip: Key 
  Summary: The set of attributes to which this constraint applies.  
  Description: The set of attributes to which this constraint applies. For a primary key or a unique key, the set of attributes must be identifying. For a foreign key, the set of attributes must match an identifying key in the referenced entity. For a not null constraint, the key indicates the attribute which should not be null.  
  Example:  
referencedEntityId
  Tooltip: Referenced Entity ID 
  Summary: The id of the parent-entity in a foreign key constraint.  
  Description: This field contains the id of the entity to which a foreign key refers, otherwise known as the parent-entity or parent-table.  
  Example: site_properties  
referencedKey
  Tooltip: Key 
  Summary: The set of attributes to which a foreign key constraint refers.  
  Description: The set of attributes to which a foreign key constraint refers. If the key refers to the primary key in the referenced entity, then the "referencedKey" is optional. For a foreign key, the set of attributes must match an identifying key in the referenced entity.  
  Example:  
cardinality
  Tooltip: Constrint Cardinality  
  Summary: The cardinality of a foreign key constraint. 
  Description: Indicates the cardinality (zero, one, many) of a foreign key constraint.  
  Example:  
checkCondition
  Tooltip: Check Condition  
  Summary: An SQL statement to be used as the condition for a check constraint.  
  Description: A SQL statement to be used as the condition for a check constraint. Generally this provides a means for constraining the values within and among entities.  
  Example: (year > 1900 and year < 1990) 

Attribute Definitions:

system
  Tooltip: Catalog system  
  Summary: The catalog system in which this identifier is used.  
  Description: This element gives the name of the catalog system in which this identifier is used. It is useful to determine the scope of the identifier, and to determine the semantics of the various subparts of the identifier. Unresolved issue: can or should this be a URI/URL pointing to the catalog system, or just the name?  
  Example: <identifier system="metacat">nceas.3.2</identifier> 

Web Contact: jones@nceas.ucsb.edu