  We accomplish for the first time the next-to-leading-order computation of the
weak annihilation contribution to the exclusive electroweak penguin decays $B
\to \left \{K, \pi \right \} \ell^{+} \ell^{-}$ with an energetic light-flavour
meson, which is an essential missing piece of the complete QCD correction to
the matrix elements of hadronic operators in the weak effective Hamiltonian.
Both the hard functions and the jet functions in the perturbative factorization
formulae from the short-distance fluctuations at the two distinct scales $m_b$
and $\sqrt{m_b \, \Lambda}$ are determined at ${\cal O}(\alpha_s)$ with the
soft-collinear factorization technique. We then demonstrate that the one-loop
weak annihilation correction can bring about the noticeable impacts on theory
predictions for the CP asymmetries and the isospin asymmetry in the $B \to \pi
\ell^{+} \ell^{-}$ decays.
