Query to merge documents within the same bucket

SQL++
2

Requires 6.5 
MERGE default AS t USING  (SELECT d.cdb_id, d.phone FROM default AS d WHERE d.gala_id IS MISSNG) AS s
ON s.cbd_id = t.cbd_id AND t.gala_id  IS NOT MISSING 
WHEN MATCHED  THEN t.phone = s.phone;