SQL - CREATE LINK

Creates a link between two simple values.

Syntax

CREATE LINK <link> TYPE [<link-type>] FROM <source-class>.<source-property> TO <destination-class>.<destination-property> [INVERSE]
  • <link> Defines the property for the link. When not expressed, the link overwrites the <destination-property> field.
  • <link-type> Defines the type for the link. In the event of an inverse relationship, (the most common), you can specify LINKSET or LINKLIST for 1-n relationships.
  • <source-class> Defines the class to link from.
  • <source-property> Defines the property to link from.
  • <destination-class> Defines the class to link to.
  • <destination-property> Defines the property to link to.
  • INVERSE Defines whether to create a connection on the opposite direction. This option is common when creating 1-n relationships from a Relational database, where they are mapped at the opposite direction.

Example

  • Create an inverse link between the classes Comments and Post:

     orientdb> CREATE LINK comments TYPE LINKSET FROM Comments.PostId TO Posts.Id 
           INVERSE

For more information, see

Conversion from Relational Databases

You may find this useful when imported data from a Relational database. In the Relational world, the database uses links to resolve foreign keys. In general, this is not the way to create links, but rather a way to convert two values in two different classes into a link.

As an example, consider a Relational database where the table Post has a 1-n relationship with the table Comment. That is Post 1 ---> * Comment, such as:

reldb> SELECT * FROM Post;

+----+----------------+
| Id | Title          |
+----+----------------+
| 10 | NoSQL movement |
+----+----------------+
| 20 | New OrientDB   |
+----+----------------+

reldb> SELECT * FROM Comment;

+----+--------+--------------+
| Id | PostID | Text         |
+----+--------+--------------+
|  0 | 10     | First        |
+----+--------+--------------+
|  1 | 10     | Second       |
+----+--------+--------------+
| 21 | 10     | Another      |
+----+--------+--------------+
| 41 | 20     | First again  |
+----+--------+--------------+
| 82 | 20     | Second Again |
+----+--------+--------------+

In OrientDB, instead of a separate table for the relationship, you use a direct relationship as your object model. Meaning that the database navigates from Post to Comment and not vice versa, as with Relational databases. To do so, you would also need to create the link with the INVERSE option.