T(ST_UNION(@g1,@g2));
+------------------------------------------------+
| ASTEXT(ST_UNION(@g1,@g2))                      |
+------------------------------------------------+
| POLYGON((0 0,0 3,2 3,2 4,4 4,4 2,3 2,3 0,0 0)) |
+------------------------------------------------+

URL: https://mariadb.com/kb/en/st_union/https://mariadb.com/kb/en/st_union/