I have a field where the most of the records is empty, how do i validate if it is empty or not?

This is mi query:

sensql="SELECT d.cod_und, d.cod_jef, d.cod_dpt, d.sig_dpt, d.des_dpt, j.sig_jef, u.sig_und, s.cod_sec, s.des_sec from m_depart d, m_jefatu j, m_unidad u,m_seccio s where j.cod_jef=d.cod_jef and j.cod_und=d.cod_und and j.cod_und=u.cod_und and d.cod_und="&rs_consulta("interfaceu")&" and d.cod_jef="&rs_consulta("interfacej")&" and d.cod_dpt="&rs_consulta("interfaced")

the problem is when i place the field to validate, so i validate it in this way but it's wrong:

if rs_consulta("interfaces")<> "" then
sensql=sensql+" and s.cod_sec="&rs_consulta("interfaces")
end if


Please help me. thanks