# ST_Union function and ST_Union aggregation function

The ST_Union function takes two geometries as input parameters and returns the geometry that is the union of the specified geometries. The resulting geometry is represented in the spatial reference system of the first geometry. Another form of this function is the ST_Union aggregate function. Use the ST_Union aggregate function to aggregate a set of geometries from a column in a table to single geometry by constructing the union.

## ST_Union function

Both geometries must be of the same dimension. If any of the two specified geometries is null, null is returned.

If the second geometry is not represented in the same spatial reference system as the first geometry and uses the same underlying datum, it will be converted to the other spatial reference system.

The resulting geometry is represented in the most appropriate spatial type. If it can be represented as a point, linestring, or polygon, then one of those types is used. Otherwise, the multipoint, multilinestring, or multipolygon type is used.

## Syntax

## Parameters

- geometry1
- A value of type ST_Geometry or one of its subtypes that is combined with
*geometry2*. - geometry2
- A value of type ST_Geometry or one of its subtypes that is combined with
*geometry1*.

## Return type

ST_Geometry

## Examples

## ST_Union aggregation function

ST_Union aggregate function returns a geometry or null depending upon the selected set of geometries.

If all of the geometries to be combined in the union are null, then null is returned. If each of the geometries to be combined in the union are either null or are empty, then an empty geometry of type ST_Point is returned.

## Prerequisites

Spatial Analytics must be enabled.

## Syntax

## Parameter

- geometry
- A column that has a type of ST_Geometry or one of its subtypes and contains the set of geometries for which the union is to be computed.

## Return type

ST_Geometry

## Restrictions

- For the ST_Union aggregation function, all the geometries in the set must belong to the same geometry type, otherwise an incompatible geometries error will be reported.
- For the ST_Union aggregation function, all the geometries in the set must have compatible spatial reference systems, otherwise an error will be reported. In case the spatial reference system (SRS) is compatible, but not the same, the SRS of the resulting geometry will be the same as the SRS of the first geometry in the set (geometries with a different SRS are converted into the first SRS).

## Examples

- Example 1:
- This example illustrates how a union aggregate can be used to combine a set of points into
multipoints. Several points are added to the SAMPLE_POINTS table. The ST_Union aggregation function
is used to construct the union of the
points:
`CREATE TABLE sample_points (id INTEGER, geometry ST_Point) INSERT INTO sample_points VALUES (1, ST_Point (2, 3, 1) ) INSERT INTO sample_points VALUES (2, ST_Point (4, 5, 1) ) INSERT INTO sample_points VALUES (3, ST_Point (13, 15, 1) ) INSERT INTO sample_pointsVALUES (4, ST_Point (12, 5, 1) ) INSERT INTO sample_pointsVALUES (5, ST_Point (23, 2, 1) ) INSERT INTO sample_points VALUES (6, ST_Point (11, 4, 1) ) SELECT SUBSTR (ST_AsText( ST_UNION(geometry) ) ,1, 200) AS POINT_AGGREGATE FROM sample_points`

Result:`POINT_AGGREGATE ---------------------------------------------------------- MULTIPOINT ( 2.00000000 3.00000000, 4.00000000 5.00000000, 11.00000000 4.00000000, 12.00000000 5.00000000, 13.00000000 15.00000000, 23.00000000 2.00000000) 1 record(s) selected.`

- Example 2:
- The following example shows the ST_Union function used in a group-by query to compute the union
of all the geometries in the respective
groups:
`CREATE TABLE sample_geometries(id int, region int, geometry st_geometry) INSERT INTO sample_geometries values (1, 1, st_point(1,1)) INSERT INTO sample_geometries values (2, 1, st_point(2,2)) INSERT INTO sample_geometries values (3, 2, st_linestring('linestring(0 0 , 2 2)')) INSERT INTO sample_geometries values (4, 2, st_linestring('linestring (1 1, 3 3)')) INSERT INTO sample_geometries values (5, 3, st_point(3,3)) INSERT INTO sample_geometries values (6, 2, st_linestring('linestring(0 1 , 2 1)')) SELECT region ,SUBSTR(ST_ASTEXT(ST_UNION(geometry)),1,200) AS "Union_of_Geos" from sample_geometries WHERE id < 5 GROUP BY region`

Result:`REGION UNION_OF_GEOS ----------- ------------------------------------------------------------------------------------------------------------------------------------------------------ 1 MULTIPOINT (1.000000000 1.000000000, 2.000000000 2.000000000) 2 MULTILINESTRING ((2.000000000 2.000000000, 3.000000000 3.000000000),(1.000000000 1.000000000, 2.000000000 2.000000000),(0.000000000 0.000000000, 1.000000000 1.000000000)) 2 record(s) selected.`