----------------------------+\n| ST_AsText(ST_ExteriorRing(ST_GeomFromText(@poly))) |\n+----------------------------------------------------+\n| LINESTRING(0 0,0 3,3 3,3 0,0 0)                    |\n+----------------------------------------------------+\n','http://dev.mysql.com/doc/refman/5.7/en/gis-polygon-property-functions.html');