ST_ToMultiPolygon() method

Syntax

geometry-expression.ST_ToMultiPolygon()

Returns

ST_MultiPolygon If the geometry-expression is of type ST_MultiPolygon, returns the geometry-expression. If the geometry-expression is a geometry collection containing only polygons, returns a multi-polygon object containing the elements of the geometry-expression. If the geometry-expression is of type ST_Polygon then return a multi-polygon value containing one element, the geometry-expression. If the geometry-expression is the empty set, returns an empty set of type ST_MultiSurface. Otherwise, raises an exception condition.

Examples

Example 1 – The following example returns the result MultiPolygon EMPTY.
```
SELECT NEW ST_GeomCollection().ST_ToMultiPolygon()
```
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 = 27
```
The following uses ST_ToMultiPolygon to change the type of the Shape column expression to ST_MultiPolygon. This example would also work with ShapeID 22, where the Shape value is of type ST_LineString. ST_Area returns the result 8.
```
SELECT Shape.ST_ToMultiPolygon().ST_Area()
FROM SpatialShapes WHERE ShapeID = 27
```
In this case, the value of the Shape column is known be of type ST_MultiPolygon, so TREAT can be used to efficiently change the type of the expression. This example would notwork with ShapeID 22, where the Shape value is of type ST_Polygon. ST_Area returns the result 8.
```
SELECT TREAT( Shape AS ST_MultiPolygon ).ST_Area()
FROM SpatialShapes WHERE ShapeID = 27
```

Standards

SQL/MM (ISO/IEC 13249-3: 2006) 5.1.33