Select Distinct

It is quite useful to give Entities common properties, like a “type”. But when filtering on such a common property the client needs to know what the used values are. This extension allows a client to request all distinct values for a field or a set of fields.

Distinct select can be used in Expands, and can be ordered. When combining $orderby with a distinct select, it is only possible to order by the exact fields that are selected.

Note that selecting distinct values for the @iot.id field makes no sense, since the ID field is unique for each entity.

Syntax and Returned Data

To request the distinct values for a set of selected fields, add the distinct: keyword at the start of the $select parameter.

The returned data is formatted just like a non-distinct request of the same type would be.

For example, the following request returns all distinct values of the properties/type field of all Things:

v1.1/Things?$select=distinct:properties/type

The returned data could be:

{
    "value": [
        {
            "properties": {
                "type": "waterBody"
            }
        },
        {
            "properties": {
                "type": "station"
            }
        },
        {
            "properties": {
                "type": "aquifer"
            }
        }
    ]
}

The following request returns all distinct combinations that exist, for the fields properties/type and properties/subType

v1.1/Things?$select=distinct:properties/type,properties/subType

The returned data could be:

{
    "value": [
        {
            "properties": {
                "subType": "river",
                "type": "waterBody"
            }
        },
        {
            "properties": {
                "subType": "Grundwasserleiter und Grundwassergeringleiter",
                "type": "aquifer"
            }
        },
        {
            "properties": {
                "subType": "Grundwasserleiter",
                "type": "aquifer"
            }
        },
        {
            "properties": {
                "subType": "Grundwassergeringleiter",
                "type": "aquifer"
            }
        },
        {
            "properties": {
                "type": "station"
            }
        }
    ]
}

Conformance Class

The conformance class this extension must register in the SensorThings (v1.1 and up) index document is:

https://fraunhoferiosb.github.io/FROST-Server/extensions/SelectDistinct.html