  The purpose of this paper is to answer the question whether it is possible to
realize simultaneously the relations $N_{\alpha,\beta}=-N_{-\alpha,-\beta}$,
$N_{\alpha,\beta}=N_{\beta,-\alpha-\beta}=N_{-\alpha-\beta,\alpha}$ and
$N_{\alpha,\beta}N_{-\alpha,-\beta}=-\frac{1}{2}q(p+1)\langle\alpha,H_{\alpha}\rangle$
by the structure constants of the Lie algebra ${\mathfrak g}_2$. We show that
if the structure constants obey the first relation, the three last ones are
violated, and vice versa. Contrary to the second case, the first one uses the
Cartan matrix elements to derive the structure constants in the form of
$\langle\beta,H_{\alpha}\rangle$. The commutation relations corresponding to
the first case are exactly documented in the prior literature. However, as
expected, a Lie algebra isomorphism is established between the Cartan-Weyl
bases obtained in both approaches.
