  We obtain some estimates for norm and essential norm of the difference of two
composition operators between weighted Bergman spaces $A^p_\alpha$ and
$A^q_\beta$ on the unit ball. In particular, we completely characterize the
boundedness and compactness of $C_\varphi-C_\psi: A^p_\alpha\to A^q_\beta$ for
full range $0<p,q<\infty$, $-1<\alpha,\beta<\infty$.
