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Returns a geometry that represents the convex hull of the geometry
value g.

This function computes a geometry's convex hull by first checking
whether its vertex points are colinear. The function returns a linear
hull if so, a polygon hull otherwise. This function processes geometry
collections by extracting all vertex points of all components of the
collection, creating a MultiPoint value from them, and computing its
convex hull.

ST_ConvexHull() handles its arguments as described in the introduction
to this section, with this exception:

o The return value is NULL for the additional condition that the
argument is an empty geometry collection.



mysql> SET @g = 'MULTIPOINT(5 0,25 0,15 10,15 25)';
mysql> SELECT ST_AsText(ST_ConvexHull(ST_GeomFromText(@g)));
| ST_AsText(ST_ConvexHull(ST_GeomFromText(@g))) |
| POLYGON((5 0,25 0,15 25,5 0)) |

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