select a_col, b_col, c_col, decode(b_col,'K1',null,'K3',c_col) d_col ...

근데 K1, K3 이외의 값은요?

select decode(b_col,'k1',null,'k3',c_col,d_col) as d_col from dual;

--case문

select case b_col when 'k1' then null when 'k3' then c_dol else d_col end d_col from dual;