用 !empty older row columns 填充空的数据库列

Question

假设我有一个包含以下行的数据库：

id (auto_incremented) | id_person | first_name | last_name | phone          | email
-----------------------------------------------------------------------------
1                     | 12        | kevin      | smith     |                | kevin@hotmail.com
2                     | 12        | kevin      | smith     | 1-800-123-4567 | 
3                     | 33        | joe        | jones     | 1-800-765-4321 | 
4                     | 33        | joe        | thompson  |                | joe@hotmail.com
5                     | 33        | joe        | thompson  |                | newjoe@hotmail.com

基于id_person，我想在单个查询中输出以下内容：

id | id_person | first_name | last_name | phone          | email
-----------------------------------------------------------------------------
2  | 12        | kevin      | smith     | 1-800-123-4567 | kevin@hotmail.com
5  | 33        | joe        | thompson  | 1-800-765-4321 | newjoe@hotmail.com

所以基本上，我只想获取最新的行值，如果它们为空，则从非空的第一行获取值。

我该怎么做？希望这是有道理的。

Answer 1

SELECT id_person, subQ.lastID
    , GROUP_CONCAT(IF(subQ.lastFieldXId = tblFields.ID, tblFields.fieldX, NULL)) AS fieldX
    , GROUP_CONCAT(IF(subQ.lastFieldYId = tblFields.ID, tblFields.fieldY, NULL)) AS fieldY
    [, ...]
FROM
(SELECT id_person
   , MAX(id) AS lastID
   , MAX(IF(IFNULL(fieldX, '') = '', 0, id) AS lastFieldXId
   , MAX(IF(IFNULL(fieldY, '') = '', 0, id) AS lastFieldYId
[, ....]
FROM theTable
GROUP BY id_person) AS subQ
LEFT JOIN theTable AS tblFields USING (id_person)
;

LEFT JOIN 可能会也可能不会更快

LEFT JOIN theTable AS tblFields 
ON tblFields.ID IN (subQ.lastFieldXId, subQ.lastFieldYId [, ...])

可能还有一种使用会话变量的方法，但我没有精力...

Answer 2

select * from(
select p1.id,p1.id_person,p1.first_name,p1.last_name,
ifnull(p1.phone,p2ph.phone) phone ,ifnull(p1.email,p3em.email) email

from person p1
left join person p2ph on(p1.id_person=p2ph.id_person and p2ph.phone is not null)
left join person p3em on(p1.id_person=p3em.id_person and p3em.email is not null)
order by id_person,p1.id desc,p2ph.id desc,p3em.id desc 
  )t group by id_person;

在 http://sqlfiddle.com/#!9/d2f44/6

验证

用 !empty older row columns 填充空的数据库列

Populate empty database columns with !empty older row columns

mysql

merge

select

rows