ST_ToPolygon method for type ST

ST_ToPolygon method for type ST_Geometry

Converts the geometry to a polygon.

Syntax

geometry-expression.ST_ToPolygon()

Returns

ST_Polygon If the geometry-expression is of type ST_Polygon, returns the geometry-expression. If the geometry-expression is of type ST_CurvePolygon, returns geometry-expression.ST_CurvePolyToPoly(). If the geometry-expression is a geometry collection with a single element of type ST_CurvePolygon, returns that element. If the geometry-expression is the empty set, returns an empty set of type ST_Polygon. Otherwise, raises an exception condition.
The spatial reference system identifier of the result is the same as the spatial reference system of the geometry-expression.

Convert the geometry to a polygon. The logic is equivalent to that used for CAST( geometry-expression AS ST_Polygon ). If the geometry-expression is a curve polygon, it is approximated using ST_CurvePolyToPoly().

If geometry-expression is already known to be an ST_Polygon value, it is more efficient to use TREAT( geometry-expression AS ST_Polygon ) than the ST_ToPolygon method.

Note

By default, ST_ToPolygon uses the original format for a geometry, if it is available. Otherwise, the internal format is used. For more information about internal and original formats, see STORAGE FORMAT clause, CREATE SPATIAL REFERENCE SYSTEM statement.

Standards and compatibility

Example

The following example returns the result Polygon EMPTY.

SELECT NEW ST_GeomCollection().ST_ToPolygon().ST_AsText()

The following returns an error because the Shape column is of type ST_Geometry and ST_Geometry does not support the ST_Area method.

SELECT Shape.ST_Area()
FROM SpatialShapes WHERE ShapeID = 22

The following uses ST_ToPolygon to change the type of the Shape column expression to ST_Polygon. ST_Area returns the result 12.5.

SELECT Shape.ST_ToPolygon().ST_Area()
FROM SpatialShapes WHERE ShapeID = 22

In this case, the value of the Shape column is known be of type ST_Polygon, so TREAT can be used to efficiently change the type of the expression. ST_Area returns the result 12.5.

SELECT TREAT( Shape AS ST_Polygon ).ST_Area() 
FROM SpatialShapes WHERE ShapeID = 22

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