A constraint defines a test element that compares the identity or attribute values of two vertices or two edges. You cannot mix vertices and edges within a constraint, but queries may include both vertex constraints and edge constraints. Because constraints apply to two different query elements, they occur at the top level within a query, rather than under any specific element. Specifically, you cannot include constraints within subquery elements. You can combine multiple constraints with and elements.

Constraints take slightly different forms depending on whether they are comparing attributes or identity. In the example below, the first test requires that query items (vertices or edge) A and B have the same value for attribute attr. The second test requires that query items B and C do not map to the same entity in the database.

Vertex and edge names used in the constraint, as specified by the item-name element, must match the name and case of vertices and edges defined for this query.

<constraint>

Attributes  None.

Children 

Content Model 

Although the DTD does not enforce this, Proximity requires that constraint test elements may only be combined with and.

Example 

<constraint>
  <and>
    <test>
      <operator>eq</operator>
      <item>
        <item-name>A</item-name>
        <attribute-name>objtype</attribute-name>
      </item>
      <item>
        <item-name>B</item-name>
        <attribute-name>objtype</attribute-name>
      </item>
    </test>
    <test>
      <operator>ne</operator>
      <item>
        <item-name>B</item-name>
        <id/>
      </item>
      <item>
        <item-name>C</item-name>
        <id/>
      </item>
    </test>
  </and>
</constraint>