ST_SymDifference(g1, g2)

Returns a geometry that represents the point set symmetric difference
of the geometry values g1 and g2, which is defined as:

g1 symdifference g2 := (g1 union g2) difference (g1 intersection g2)

Or, in function call notation:

ST_SymDifference(g1, g2) = ST_Difference(ST_Union(g1, g2), ST_Intersection(g1, g2))

If any argument is NULL, the return value is NULL.

URL: https://dev.mysql.com/doc/refman/5.7/en/spatial-operator-functions.html

