documentclass[11pt,letterpaper]{memoir}
usepackage{listings}

usepackage{accsupp}

lstset{
     frameround=fttt
    ,language=SQL
    ,numbers=left
    ,breaklines=true
    ,showstringspaces=false
    ,basicstyle=small
    ,numberstyle=noncopynumber
    ,columns=flexible
}

newcommand{noncopynumber}[1]{
    BeginAccSupp{method=escape,ActualText={}}
    #1
    EndAccSupp{}
}

lstloadlanguages{SQL}

begin{document}
begin{lstlisting}
declare @a int

if @a = null
  select 'null' as outcome
else
  select 'not null' as outcome
end{lstlisting}
end{document}

declare @a int
if @a = null
select ' null ' as outcome
else
select 'not null ' as outcome
if null = null
select 'equal' as outcome
else
select 'not equal' as outcome