STConvexHull (geometry Data Type)
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
Returns an object representing the convex hull of a geometry instance.
Syntax
.STConvexHull ( )
Return Types
SQL Server return type: geometry
CLR return type: SqlGeometry
Remarks
STConvexHull() returns the smallest convex polygon that contains the given geometry instance. Points or co-linear LineString instances will produce an instance of the same type as that of the input.
Examples
The following example uses STConvexHull() to find the convex hull of a non-convex Polygon``geometry instance.
DECLARE @g geometry;
SET @g = geometry::STGeomFromText('POLYGON((0 0, 0 2, 1 1, 2 2, 2 0, 0 0))', 0);
SELECT @g.STConvexHull().ToString();