pool.ntp.sqldb.us has no NS records.
NS records for this zone are defined at sqldb.us.
NS records for this zone are defined at sqldb.us.
Question and response
Question
dig pool.ntp.sqldb.us. NS
Answer
pool.ntp.sqldb.us. 5 CNAME casjay.pool.ntp.org.
Authority
pool.ntp.org. 1300 SOA h.ntpns.org. hostmaster.pool.ntp.org. 1772528134 5400 5400 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0