ST_Union

Definition

ST_Union returns an ST_Geometry object that is the combination of two source objects.

Syntax

sde.st_union (g1 sde.st_geometry, g2 sde.st_geometry)

Return type

ST_Geometry

Example

The sensitive_areas table stores the IDs of threatened institutions in addition to the shape column, which stores the institutions' ST_Polygon geometries.

The hazardous_sites table stores the identity of the sites in the id column, while the actual geographic location of each site is stored in the site point column.

CREATE TABLE sensitive_areas (id integer, 
                              shape sde.st_geometry); 

CREATE TABLE hazardous_sites (id integer, 
                              site sde.st_geometry);


INSERT INTO SENSITIVE_AREAS VALUES (
1,
sde.st_geometry ('polygon ((20 30, 30 30, 30 40, 20 40, 20 30))', 0)
);

INSERT INTO SENSITIVE_AREAS VALUES (
2,
sde.st_geometry ('polygon ((30 30, 30 50, 50 50, 50 30, 30 30))', 0)
);

INSERT INTO SENSITIVE_AREAS VALUES (
3,
sde.st_geometry ('polygon ((40 40, 40 60, 60 60, 60 40, 40 40))', 0)
);

INSERT INTO HAZARDOUS_SITES VALUES (
4,
sde.st_geometry ('point (60 60)', 0)
);

INSERT INTO HAZARDOUS_SITES VALUES (
5,
sde.st_geometry ('point (30 30)', 0)
);

The ST_Buffer function generates a buffer surrounding the hazardous waste sites. The ST_Union function generates polygons from the union of the buffered hazardous waste sites and sensitive area polygons. The ST_Area function returns the area of these polygons.

Oracle

SELECT sa.id SA_ID, hs.id HS_ID,
sde.st_area (sde.st_union (sde.st_buffer (hs.site, .01), sa.shape)) UNION_AREA
FROM HAZARDOUS_SITES hs, SENSITIVE_AREAS sa;

     SA_ID  HS_ID     UNION_AREA

         1    4       100.000313935011
         2    4       400.000313935011
         3    4       400.000235451258
         1    5       100.000235451258
         2    5       400.000235451258
         3    5       400.000313935011

PostgreSQL

SELECT sa.id AS SA_ID, hs.id AS HS_ID,
sde.st_area (sde.st_union (sde.st_buffer (hs.site, .01), sa.shape)) AS UNION_AREA
FROM hazardous_sites hs, sensitive_areas sa;

     sa_id  hs_id   union_area

         1    4       100.000313935011
         2    4       400.000313935011
         3    4       400.000235451258
         1    5       100.000235451258
         2    5       400.000235451258
         3    5       400.000313935011