ST_Boundary - Amazon Redshift
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ST_Boundary

ST_Boundary returns the boundary of an input geometry as follows:

• If the input geometry is empty (that is, it contains no points) it is returned as is.

• If the input geometry is a point or nonempty multipoint, an empty geometry collection is returned.

• If the input is a linestring or a multilinestring, then a multipoint containing all the points on the boundary is returned. The multipoint might be empty).

• If the input is a polygon that does not have any interior rings, then a closed linestring representing its boundary is returned.

• If the input is a polygon that has interior rings, or is a multipolygon, then a multilinestring is returned. The multilinestring contains all the boundaries of all the rings in the areal geometry as closed linestrings.

To determine point equality, ST_Boundary operates on the 2D projection of the input geometry. If the input geometry is empty, a copy of it is returned in the same dimension as the input. For nonempty 3DM and 4D geometries, their `m` coordinates are dropped. In the special case of 3DZ and 4D multilinestrings, the `z` coordinates of the multilinestring's boundary points are computed as the averages of the distinct z-values of the linestring boundary points with the same 2D projection.

Syntax

``ST_Boundary(geom)``

Arguments

geom

A value of data type `GEOMETRY` or an expression that evaluates to a `GEOMETRY` type.

Return type

`GEOMETRY`

If geom is null, then null is returned.

If geom is a `GEOMETRYCOLLECTION`, then an error is returned.

Examples

The following SQL returns the boundary of the input polygon as a multilinestring.

``SELECT ST_AsEWKT(ST_Boundary(ST_GeomFromText('POLYGON((0 0,10 0,10 10,0 10,0 0),(1 1,1 2,2 1,1 1))')));``
``````
st_asewkt
--------------------
MULTILINESTRING((0 0,10 0,10 10,0 10,0 0),(1 1,1 2,2 1,1 1))
``````