MySQL:

ST_UNION

ST_Union(g1, g2)

Returns a geometry that represents the point set union of the geometry
values g1 and g2.

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

Example

mysql> SET @g1 = ST_GeomFromText('LineString(1 1, 3 3)');
mysql> SET @g2 = ST_GeomFromText('LineString(1 3, 3 1)');
mysql> SELECT ST_AsText(ST_Union(@g1, @g2));
+--------------------------------------+
| ST_AsText(ST_Union(@g1, @g2)) |
+--------------------------------------+
| MULTILINESTRING((1 1,3 3),(1 3,3 1)) |
+--------------------------------------+