mysql如何通过当前排序字段获取相邻数据项

( SELECT * FROM [表名] WHERE id < 40 ORDER BY id DESC LIMIT 1 ) UNION
(
    SELECT
        * 
    FROM
        [表名]
    WHERE
        id > 40 
    ORDER BY
        id 
    LIMIT 1
)

SELECT
	m.customer_id,
	sfo.created_at as '首单时间',
	m.created_at as '第二单时间',
	(unix_timestamp(m.created_at) - unix_timestamp(sfo.created_at))/86400 as '两单相差天数'
	
FROM
	sales_flat_order m 
LEFT JOIN 
	sales_flat_order sfo on m.customer_id = sfo.customer_id and sfo.created_at < m.created_at
WHERE
	(
		SELECT
			count(*)
		FROM
			sales_flat_order n
		WHERE
			m.customer_id = n.customer_id
			AND m.created_at > n.created_at
	) = 1
GROUP BY m.customer_id

SELECT
	m.customer_id,
	m.created_at as '后一单时间',
	SUBSTRING_INDEX(
		GROUP_CONCAT(sfo.created_at ORDER BY sfo.created_at DESC),
		',',
		1
	) as '前一单时间',
	(unix_timestamp(m.created_at) - unix_timestamp(
		SUBSTRING_INDEX(
			GROUP_CONCAT(sfo.created_at ORDER BY sfo.created_at DESC),
			',',
			1
		)
	))/86400 as '两单相差天数'
FROM
	sales_flat_order m 
LEFT JOIN 
	sales_flat_order sfo on m.customer_id = sfo.customer_id and sfo.created_at < m.created_at
WHERE
	(
		SELECT
			count(*)
		FROM
			sales_flat_order n
		WHERE
			m.customer_id = n.customer_id
			AND m.created_at > n.created_at
	) = 2
GROUP BY m.customer_id;