選択に関する1対1の明確な制限

試してみてください：

select a.id, a.x as ax, b.x as bx, x.min_abs_diff
  from table_a a
  join table_b b
    on a.id = b.id
  join (select a.id, min(abs(a.x - b.x)) as min_abs_diff
          from table_a a
          join table_b b
            on a.id = b.id
         group by a.id) x
    on x.id = a.id
   and abs(a.x - b.x) = x.min_abs_diff

フィドル： http://sqlfiddle.com/#!15/ab5ae/5/0

期待される出力とは一致しませんが、各ペアには絶対値1の違いがあることがわかるので、説明した内容に基づいて出力は正しいと思います。

編集-aからbの順序に基づいて、以下を試してください：

select *
  from (select a.id,
               a.x as ax,
               b.x as bx,
               x.min_abs_diff,
               row_number() over(partition by a.id, b.x order by a.id, a.x) as rn
          from table_a a
          join table_b b
            on a.id = b.id
          join (select a.id, min(abs(a.x - b.x)) as min_abs_diff
                 from table_a a
                 join table_b b
                   on a.id = b.id
                group by a.id) x
            on x.id = a.id
           and abs(a.x - b.x) = x.min_abs_diff) x
 where x.rn = 1

フィドル： http://sqlfiddle.com/#!15/ab5ae/19/0