named-sdb: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/sbin/named-sdb - from package: bind-sdb

You may install the required package with command yum install {package}

Error in other languages:
named-sdb: bevel nie gevind nie
named-sdb: comandă negăsită
named-sdb: comando não encontrado
named-sdb: commande introuvable
named-sdb: command not found
named-sdb: käsku ei ole
named-sdb: không tìm thấy lệnh
named-sdb: komanda nerasta
named-sdb: Kommando nicht gefunden.
named-sdb: kommandot finns inte
named-sdb: komut yok
named-sdb: nie znaleziono polecenia
named-sdb: níor aimsíodh an t-ordú
named-sdb: no se encontró la orden
named-sdb: no s'ha trobat l'ordre
named-sdb: opdracht niet gevonden
named-sdb: parancs nem található
named-sdb: perintah tidak ditemukan
named-sdb: príkaz nenájdený
named-sdb: příkaz nenalezen
named-sdb: команда не найдена
named-sdb: командата не е открита
named-sdb: 命令找不到
named-sdb: コマンドが見つかりません