  We study the initial value problem of the quadratic nonlinear Schr\"odinger
equation $$ iu_t+u_{xx}=u\bar{u}, $$ where $u:\R\times \R\to \C$. We prove that
it's locally well-posed in $H^s(\R)$ when $s\geq -\dfrac{1}{4}$ and ill-posed
when $s< -\dfrac{1}{4}$, which improve the previous work in \cite{KPV}.
Moreover, we consider the problem in the following space, $$
H^{s,a}(\R)={u:\|u\|_{H^{s,a}}\triangleq (\displaystyle\int
(|\xi|^s\chi_{\{|\xi|>1\}}+|\xi|^a\chi_{\{|\xi|\leq 1\}})^2|\hat{u}(\xi)|^2
d\xi)^{{1/2}}<\infty} $$ for $s\leq 0, a\geq 0$. We establish the local
well-posedness in $H^{s,a}(\R)$ when $s\geq -\dfrac{1}{4}-\dfrac{1}{2}a$ and
$a<\dfrac{1}{2}$. Also we prove that it's ill-posed in $H^{s,a}(\R)$ when
$s<-\dfrac{1}{4}-\dfrac{1}{2}a$ or $a>\dfrac{1}{2}$. It remains the cases on
the line segment: $a=\dfrac{1}{2}$, $-\dfrac{1}{2}\leq s\leq 0$ open in this
paper.
