---

NOTE

The information contained in this document represents information about preview features of the product. Features might change when the product is released for general availability.

---

Relationships

Relationships in the Gaia Database allow a row in one table to reference one or more rows in another table. There are two ways to create a relationship between tables:

• Explicit Relationships
• Implicit Relationships

Explicit Relationships

To create an Explicit Relationship you link the rows using the connect() method. To unlink the row you call the disconnect() method. The relationship is considered explicit because you specify links between specific rows in the database, independently of the data in those rows.

Implicit Relationships

To associate two tables with an Implicit Relationship, specify a Value Linked Reference in the Data Definition Language (DDL) file. When two rows in different tables contain the same value in their linked fields, a link is automatically created between the rows and a relationship is created between the tables. The relationship is considered implicit because links between rows are data-dependent: two rows are linked if the values in their linked fields are identical.

For more information on Implicit Relationships, see Implicit Relationships.

create

Creates a one-to-one or a one-to-many link between tables.

[create] relationship [if not exists] relationship_name (
    table_name_1.field_name_1 -> table_name_2[],
    table_name_2.field_name_2 -> table_name_1
    [, USING table_name_2(field_name_3,...), table_name_1(field_name_4, ...)]
);

options

if not exists: Specifies that the relationship is to be created only if a relationship of the specified name doesn't already exist. using: Indicates the relationship in the referenced table for the current relationship definition to match. It is only needed when you have more than one relationship between the two specified tables.

The short form of the create relationship statement, in which you omit the create operator, drops the table before attempting to create it and has the same effect as the following:

drop relationship if exists foo; create relationship relationship_name;

drop

drop relationship_name;

Removes the specified relationship from the catalog.

Remarks

The relationship statement adds virtual fields to each of the specified tables.

The following examples create relationships named "incubator_sensors" and "incubator_actuators".

create relationship if not exists incubator_sensors (
    incubator.sensors -> sensor[],
    sensor.incubator -> incubator
);
 
create relationship if not exists incubator_actuators (
    incubator.actuators -> actuator[],
    actuator.incubator -> incubator
);

In incubator_sensors the incubator.sensors -> sensor[] statement adds a virtual field to the incubators table that links to the sensor.incubator -> incubator statement adds a virtual field to the sensor table that links back to the incubator table.

To create a one to one relationship omit the brackets ([]).

incubator.sensors -> sensor

Use the interactive feature of gaiac to list the instantiated relationships:

gaiac> \lr
+----------------------------------+--------------------------+--------------------------------------------+----+
| Name                             | Parent (link)            | Child (link)                               | ID |
+----------------------------------+--------------------------+--------------------------------------------+----+
| gaia_catalog_database_table      | gaia_database (database) | gaia_table (gaia_tables)                   | 10 |
+----------------------------------+--------------------------+--------------------------------------------+----+
| gaia_catalog_table_field         | gaia_table (table)       | gaia_field (gaia_fields)                   | 19 |
+----------------------------------+--------------------------+--------------------------------------------+----+
| gaia_catalog_relationship_parent | gaia_table (parent)      | gaia_relationship (outgoing_relationships) | 32 |
+----------------------------------+--------------------------+--------------------------------------------+----+
| gaia_catalog_relationship_child  | gaia_table (child)       | gaia_relationship (incoming_relationships) | 33 |
+----------------------------------+--------------------------+--------------------------------------------+----+
| gaia_catalog_ruleset_rule        | gaia_ruleset (ruleset)   | gaia_rule (gaia_rules)                     | 42 |
+----------------------------------+--------------------------+--------------------------------------------+----+
| gaia_catalog_table_index         | gaia_table (table)       | gaia_index (gaia_indexes)                  | 48 |
+----------------------------------+--------------------------+--------------------------------------------+----+
| incubator_sensors                | incubator (incubator)    | sensor (sensors)                           | 72 |
+----------------------------------+--------------------------+--------------------------------------------+----+
| incubator_actuators              | incubator (incubator)    | actuator (actuators)                       | 73 |
+----------------------------------+--------------------------+--------------------------------------------+----+