  The solvability in Sobolev spaces is proved for divergence form second order
elliptic equations in the whole space, a half space, and a bounded Lipschitz
domain. For equations in the whole space or a half space, the leading
coefficients $a^{ij}$ are assumed to be measurable in one direction and have
small BMO semi-norms in the other directions. For equations in a bounded
domain, additionally we assume that $a^{ij}$ have small BMO semi-norms in a
neighborhood of the boundary of the domain. We give a unified approach of both
the Dirichlet boundary problem and the conormal derivative problem. We also
investigate elliptic equations in Sobolev spaces with mixed norms under the
same assumptions on the coefficients.
