This element tree is found at (XPath):

The <constraint> tree is for describing any integrity constraints between entities within a data package (e.g. tables), as they would be maintained in a relational management system. Use of the <constraint> tree is encouraged when data elements contain integrity constraints from a relational database. Example TO-DO shows the constraints for the <attributeList> in Example TO-DO. If there are constraints in which several columns are involved, these should be described in methods/qualityControl, since EML is not currently equipped to handle keys defined by multiple columns. When the <constraint> tree is used, all of the entities that may be referenced should be in the same package. There are six child elements:

<primaryKey> is an element which declares the primary key in the entity to which the defined constraint pertains.

<uniqueKey> is an element which represents a unique key within the referenced entity. This is different from a primary key in that it does not form any implicit foreign key relationships to other entities; however it is required to be unique within the entity.

<nonNullConstraint> defines a constraint that indicates that no null values should be present for an attribute in this entity.

<checkConstraint> defines a constraint which checks a conditional clause within an entity.

<foreignKey> defines an SQL statement or other language implementation of the condition for a check constraint. Generally this provides a means for constraining the values within and among entities. It also provides the means to meaningfully link table for explanation of codes (de-normalization).

<joinCondition> defines a foreign key relationship among entities which relates this entity to another’s primary key.

The <primaryKey>, <uniqueKey>, <nonNullConstraint> require an additional <key> tag defining the attribute to which this constraint applies, referenced by its id attribute (described in another area). All <ConstraintType> entities require additional <constraintName> and <attributeReference> tags.

Example 24: constraint

<constraint id="soil_chemistry.PRIMARY">
<constraint id="soil_chemistry.FK_soil_chemistry_sites">