MySQLUNIONでのステートメント評価と変数割り当ての順序

@colの割り当てを他のクエリと統合しないでください。

@colに値を割り当てるための1つのクエリと、そのレコードを結果に含めるための別のクエリがあります。

SELECT @col := col AS col    -- Fetch particular record given a value of
  FROM tbl                   -- "col", assigning the identifier to @col.
 WHERE col = 'd'



SELECT col                   -- Now include the above record in the
  FROM tbl                   -- Final result-set
WHERE col = @col

UNION ALL

SELECT col                   -- Fetch the immediately preceding record,
  FROM (  SELECT col         -- ordered by "col"
            FROM tbl
           WHERE col < @col
        ORDER BY col DESC
          LIMIT 1) preceding

UNION ALL

SELECT col                   -- Fetch the immediately following record,
  FROM (  SELECT col         -- ordered by "col"
            FROM tbl
           WHERE col > @col
        ORDER BY col ASC
          LIMIT 1) following
ORDER BY col ASC;