STCentroid (geometry Data Type)
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
Returns the geometric center of a geometry instance that consists of one or more polygons.
Syntax
.STCentroid ( )
Return Types
SQL Server return type: geometry
CLR return type: SqlGeometry
Open Geospatial Consortium (OGC) type: Point
Remarks
STCentroid() returns null if the geometry instance is not a Polygon, CurvePolygon, or MultiPolygon type.
Examples
A. Computing the centroid of a Polygon instance
The following example uses STCentroid() to compute the centroid of a polygon``geometry instance:
DECLARE @g geometry;
SET @g = geometry::STGeomFromText('POLYGON((0 0, 3 0, 3 3, 0 3, 0 0),(2 2, 2 1, 1 1, 1 2, 2 2))', 0);
SELECT @g.STCentroid().ToString();
B. Computing the centroid of a CurvePolygon instance
The following example computes the centroid for a CurvePolygon instance:
DECLARE @g geometry = 'CURVEPOLYGON(CIRCULARSTRING(0 4, 4 0, 8 4, 4 8, 0 4), CIRCULARSTRING(2 4, 4 2, 6 4, 4 6, 2 4))';
SELECT @g.STCentroid().ToString() AS Centroid