Access methods

If you have done previous tutorials, then you must have seen some of the spatial access methods already.

E.g. ST_Dimension() returned the dimension of a geometry object and could be applied to a geometry of any type - be it a point or a polygon. ST_X() returns the X coordinate of an ST_POINT and works with points only.

Some other methods that can be applied to all types of geometries are presented below.

ST_XMin(), ST_XMax(), ST_YMin(), ST_YMax all retrieve corresponding minimum or maximum values of geometry’s coordinates X and Y. There are similar methods available for coordinates Z and M.

In case of a point both minimum and maximum values will be the same and equal to the same value as point-only methods retrieving coordinate value, like ST_X().

SELECT
"SHAPE".ST_asWKT() AS "WKT",
"SHAPE".ST_X() AS "POINT_X",
"SHAPE".ST_XMin() AS "POINT_Xmin",
"SHAPE".ST_XMax() AS "POINT_Xmax"
FROM "TUTORIAL_GEO"."SPATIALSHAPES"
WHERE "SHAPE".ST_GeometryType()='ST_Point';

After running the SQL statement above you should notice is that these methods return NULL when applied to an empty geometry.

The other access method you used here was ST_GeometryType() to check and filter types of geometries.

You used ST_GeometryType() in the previous step as a predicate to select only records for points. Otherwise trying to apply ST_X() to a string or polygon would return run-time error during the SQL execution.

There are some access methods that can be used only with line strings. For example:

• ST_StartPoint() and ST_EndPoint() to retrieve an ST_Point values of the starting and the ending points,
• ST_NumPoints() to get the total number of points; then it can be applied to retrieve Nth point using ST_PointN(). Numbering starts from 1 and ST_PointN() returns null in Nth point does not exist.

SELECT
"SHAPE".ST_asWKT() AS "WKT",
"SHAPE".ST_StartPoint().st_asWKT() AS "START_WKT",
"SHAPE".ST_EndPoint().st_asWKT() AS "END_WKT",
"SHAPE".ST_NumPoints() AS "NUM_POINTS",
"SHAPE".ST_PointN(2).st_asWKT()  AS "POINT2_WKT"
FROM "TUTORIAL_GEO"."SPATIALSHAPES"
WHERE "SHAPE".ST_GeometryType()='ST_LineString';

Similarly to points in strings you can get a number of individual geometries in collections (types ST_MultiPoint, ST_MultiLineString, ST_MultiPolygon) using ST_NumGeometries(). And then access each of them using ST_GeometryN() method.

SELECT
ST_UnionAggr("SHAPE").ST_GeometryType() AS "GEOM_TYPE",
ST_UnionAggr("SHAPE").ST_NumGeometries() AS "NUM_GEOMS"
FROM "TUTORIAL_GEO"."SPATIALSHAPES";

The query above aggregates all different geometries from the SPATIALSHAPES table. The result is ST_GeometryCollection with 5 different geometries in it. Some of overlapping polygons got aggregated into new single polygons, so the total number of geometries in the collection is smaller than a number of single geometries in the table.

SELECT
"SHAPE".ST_GeometryType() AS "GEOM_TYPE",
ST_UnionAggr("SHAPE").ST_GeometryType() AS "AGGR_GEOM_TYPE",
ST_UnionAggr("SHAPE").ST_NumGeometries() AS "AGGR_GEOM_NUM",
ST_UnionAggr("SHAPE").ST_GeometryN(1).ST_asWKT() AS "AGGR_GEOM_1_WKT"
FROM "TUTORIAL_GEO"."SPATIALSHAPES"
GROUP BY "SHAPE".ST_GeometryType();

This query now produced 3 different collections. Each of them contains only geometries of the same types thanks to GROUP BY statement.

You will learn more about different kinds of spatial aggregations in a separate tutorial.