Conditions for Casts to MultiPolygon order they appear in the expression. If the expression contains any polygons with inner rings, an ER_WRONG_PARAMETERS_TO_STORED_FCT  error occurs. •If the expression to cast is a well-formed geometry of type  GeometryCollection  containing only linestrings, the function result is a  MultiLineString  containing those linestrings. If the expression is empty or contains other geometry types, an  ER_INVALID_CAST_TO_GEOMETRY  error occurs. •If the expression to cast is a well-formed geometry of type other than  LineString , Polygon ,  MultiLineString ,  MultiPolygon , or  GeometryCollection , an ER_INVALID_CAST_TO_GEOMETRY  error occurs. •If the expression to cast is a well-formed geometry of type  Polygon , the function result is a MultiPolygon  containing the  Polygon  as its sole element. •If the expression to cast is a well-formed geometry of type  MultiLineString  where all elements are rings, the function result is a  MultiPolygon  containing a  Polygon  with only an outer ring for each element of the expression. If any element is not a ring, an  ER_INVALID_CAST_TO_GEOMETRY These conditions apply when the cast result type is  MultiPolygon : Conditions for Casts to MultiPolygon 2244