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Query Spatial Data for Nearest Neighbor

A common query used with spatial data is the Nearest Neighbor query. Nearest Neighbor queries are used to find the closest spatial objects to a specific spatial object. For example a store locater for a Web site often must find the closest store locations to a customer location.

A Nearest Neighbor query can be written in a variety of valid query formats, but for the Nearest Neighbor query to use a spatial index the following syntax must be used.

SELECT TOP ( number )
        [ WITH TIES ]
        [ * | expression ] 
        [, ...]
    FROM spatial_table_reference, ... 
        [ WITH 
            ( 
                [ INDEX ( index_ref ) ] 
                [ , SPATIAL_WINDOW_MAX_CELLS = <value>] 
                [ ,... ] 
            ) 
        ]
    WHERE 
        column_ref.STDistance ( @spatial_ object ) 
            { 
                [ IS NOT NULL ] | [ < const ] | [ > const ] 
                | [ <= const ] | [ >= const ] | [ <> const ] ] 
            }
            [ AND { other_predicate } ] 
    }
    ORDER BY column_ref.STDistance ( @spatial_ object ) [ ,...n ]
[ ; ] 

In SQL Server, TOP and ORDER BY clauses are used to perform a Nearest Neighbor query on spatial data columns. The ORDER BY clause contains a call to the STDistance() method for the spatial column data type. The TOP clause indicates the number of objects to return for the query.

The following requirements must be met for a Nearest Neighbor query to use a spatial index:

  1. A spatial index must be present on one of the spatial columns and the STDistance() method must use that column in the WHERE and ORDER BY clauses.

  2. The TOP clause cannot contain a PERCENT statement.

  3. The WHERE clause must contain a STDistance() method.

  4. If there are multiple predicates in the WHERE clause then the predicate containing STDistance() method must be connected by an AND conjunction to the other predicates. The STDistance() method cannot be in an optional part of the WHERE clause.

  5. The first expression in the ORDER BY clause must use the STDistance() method.

  6. Sort order for the first STDistance() expression in the ORDER BY clause must be ASC.

  7. All the rows for which STDistance returns NULL must be filtered out.

Caution note Caution

Methods that take geography or geometry data types as arguments will return NULL if the SRIDs are not the same for the types.

It is recommended that the new spatial index tessellations be used for indexes used in Nearest Neighbor queries. For more information on spatial index tessellations, see Spatial Data (SQL Server).

The following code example shows a Nearest Neighbor query that can use a spatial index. The example uses the Person.Address table in AdventureWorks2012 database.

USE AdventureWorks2012
GO
DECLARE @g geography = 'POINT(-121.626 47.8315)';
SELECT TOP(7) SpatialLocation.ToString(), City FROM Person.Address
WHERE SpatialLocation.STDistance(@g) IS NOT NULL
ORDER BY SpatialLocation.STDistance(@g);

Create a spatial index on the column SpatialLocation to see how a Nearest Neighbor query uses a spatial index. For more information on creating spatial indexes, see Create, Modify, and Drop Spatial Indexes.

The following code example shows a Nearest Neighbor query that cannot use a spatial index.

USE AdventureWorks2012
GO
DECLARE @g geography = 'POINT(-121.626 47.8315)';
SELECT TOP(7) SpatialLocation.ToString(), City FROM Person.Address
ORDER BY SpatialLocation.STDistance(@g);

The query lacks a WHERE clause that uses STDistance() in a form specified in the syntax section so the query cannot use a spatial index.

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