STEnvelope (geometry Data Type)
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
SQL database in Microsoft Fabric
Returns the minimum axis-aligned bounding rectangle of the instance.
Syntax
STEnvelope ( )
Return Types
SQL Server return type: geometry
CLR return type: SqlGeometry
Examples
The following example uses STGeomFromText() to create a LineString instance from (0,0) to (2,3), and uses STEnvelope() to return the bounding box of the LineString.
DECLARE @g geometry;
SET @g = geometry::STGeomFromText('LINESTRING(0 0, 2 3)', 0);
SELECT @g.STEnvelope().ToString();