ST_GeomCollection
If the geometry-expression is of type ST_GeomCollection, returns the geometry-expression.
If the geometry-expression is of type ST_Point, ST_Curve, or ST_Surface, then return a geometry collection containing one element, the geometry-expression.
If the geometry-expression is the empty set, returns an empty set of type ST_GeomCollection.
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.
If geometry-expression is already known to be an ST_GeomCollection value, it is more
efficient to use TREAT( geometry-expression AS ST_GeomCollection ) than the
ST_ToGeomColl method.