ST_Union(g1, g2)

Returns a geometry that represents the point set union of the geometry
values g1 and g2. The result is in the same SRS as the geometry
arguments.

ST_Union() permits arguments in either a Cartesian or a geographic SRS,
and handles its arguments as described in the introduction to this
section.

URL: https://dev.mysql.com/doc/refman/8.4/en/spatial-operator-functions.html

