ST_IsSimple(g)

Returns 1 if the geometry value g has no anomalous geometric points,
such as self-intersection or self-tangency. ST_IsSimple() returns 0 if
the argument is not simple, and NULL if the argument is NULL.

The description of each instantiable geometric class given earlier in
the chapter includes the specific conditions that cause an instance of
that class to be classified as not simple. (See [HELP Geometry
hierarchy].)

URL: http://dev.mysql.com/doc/refman/5.7/en/gis-general-property-functions.html

