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.

As of MySQL 8.0.26, ST_Union() permits arguments in either a Cartesian
or a geographic SRS. Prior to MySQL 8.0.26, ST_Union() permits
arguments in a Cartesian SRS only; for arguments in a geographic SRS,
an ER_NOT_IMPLEMENTED_FOR_GEOGRAPHIC_SRS
(https://dev.mysql.com/doc/mysql-errors/8.0/en/server-error-reference.h
tml#error_er_not_implemented_for_geographic_srs) error occurs.

ST_Union() handles its arguments as described in the introduction to
this section.

URL: https://dev.mysql.com/doc/refman/8.0/en/spatial-operator-functions.html

