SQL to know the free space available to the database

SELECT ORX.database::nvarchar(64) AS "DatabaseName" ,
case when sum(SOD.used_bytes) is null then 0 else SUM(SOD.used_bytes)/1073741824 end AS "UseSpace_GB",
case when sum(SOD.allocated_bytes) is null then 0 else SUM(SOD.allocated_bytes)/1073741824 end AS "AllocatedSpace_GB",
current_timestamp as "CurrentTime"
FROM _V_SYS_OBJECT_DSLICE_INFO SOD INNER JOIN _V_OBJ_RELATION_XDB ORX ON ORX.objid = SOD.tblid
GROUP BY "CurrentTime", "DatabaseName"
ORDER BY "DatabaseName"