Extracting Specific Geometries from Collections in MariaDB with ST_GEOMETRYN

What is ST_GEOMETRYN?

ST_GEOMETRYN is a spatial function used in MariaDB to extract a specific element (geometry) from a collection of geometries. It's particularly useful when working with data stored in the Geometry data type, which allows representing various spatial objects like points, lines, and polygons.

How does it work?

ST_GEOMETRYN takes two arguments:

1. gc (geometry collection)
   This is the input geometry collection from which you want to extract a specific element.
2. N (integer)
   This represents the index of the geometry you want to extract. Numbering starts from 1, meaning N=1 fetches the first element in the collection.

What does it return?

The function returns the N-th geometry as a standalone geometry object. This allows you to perform further spatial operations on that specific element.

Example

Suppose you have a table named locations with a column geom storing a MULTIPOINT geometry collection representing multiple points of interest. You can use ST_GEOMETRYN to extract the second point:

SELECT ST_GEOMETRYN(geom, 2) AS second_point
FROM locations;

This query will return the second point from each MULTIPOINT geometry in the geom column as a separate geometry object in the result set, aliased as second_point.

• To determine the total number of geometries in a collection, you can use the ST_NumGeometries function.
• Make sure the N value corresponds to a valid element within the geometry collection. Otherwise, the function might return NULL.
• MariaDB also offers a synonym for ST_GEOMETRYN - GeometryN(). Both function names achieve the same result.

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Extracting a LINESTRING from a MULTILINESTRING

SELECT ST_GEOMETRYN(geom, 1) AS first_linestring
FROM roads;

This query assumes a table named roads with a geom column containing MULTILINESTRING geometries representing multiple road segments. It extracts the first LINESTRING from each collection and assigns the alias first_linestring.

Selecting specific points based on index

SELECT ST_GEOMETRYN(geom, point_index) AS specific_point
FROM landmarks, point_indices
WHERE landmarks.id = point_indices.landmark_id;

This example involves two tables: landmarks storing points of interest (with geometries in the geom column) and point_indices containing an id for each landmark and a point_index indicating which point within the landmark's geometry collection to select. The query utilizes a JOIN between the tables and extracts the point specified by point_index using ST_GEOMETRYN.

Checking for empty geometries

SELECT id, 
       CASE WHEN ST_IsEmpty(ST_GEOMETRYN(geom, 1)) THEN 'Empty' ELSE 'Not Empty' END AS geometry_status
FROM parcels;

This query assumes a table named parcels with a geom column potentially containing empty geometries (representing areas with no defined spatial extent). It uses ST_GEOMETRYN to extract the first element (assuming there should be at least one) and then checks if it's empty using the ST_IsEmpty function. The result categorizes each parcel as "Empty" or "Not Empty" based on the extracted geometry.

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Looping through the collection (procedural approach)

If you need to process all elements within a geometry collection, you can achieve this by looping through them using procedural constructs within MariaDB, like iterators or user-defined functions (UDFs). This approach requires more complex code compared to ST_GEOMETRYN but offers greater flexibility for bulk processing.

Subqueries and conditional logic

For simpler scenarios, you might be able to achieve the desired outcome by utilizing subqueries and conditional logic within your main SQL statement. This could involve filtering based on specific criteria within the collection or extracting the first/last element without needing individual access.

External GIS libraries (advanced)

For advanced spatial analysis tasks, consider using external GIS libraries like PostGIS, which can be integrated with MariaDB. These libraries offer a wider range of spatial functions, including alternatives for element extraction from collections. However, this approach requires additional setup and potentially alters your workflow.

• Your comfort level with procedural programming in MariaDB
• Whether you need to process individual elements or the entire collection
• The complexity of your task