STDistance (geometry Data Type)
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
Returns the shortest distance between a point in a geometry instance and a point in another geometry instance.
Syntax
.STDistance ( other_geometry )
Arguments
other_geometry
Is another geometry instance from which to measure the distance between the instance on which STDistance() is invoked. If other_geometry is an empty set, STDistance() returns null.
Return Types
SQL Server return type: float
CLR return type: SqlDouble
Remarks
STDistance() always returns null if the spatial reference IDs (SRIDs) of the geometry instances do not match.
Examples
DECLARE @g geometry;
DECLARE @h geometry;
SET @g = geometry::STGeomFromText('POLYGON((0 0, 2 0, 2 2, 0 2, 0 0))', 0);
SET @h = geometry::STGeomFromText('POINT(10 10)', 0);
SELECT @g.STDistance(@h);