mysql - 如何根据另一列的值获取单行值？

Question

我正在尝试根据 name 列中的值从 value 列中选择值。

例如:

mysql> select name, value from rss_feed_property_value where rss_feed_id = 31;
+-------------+---------------+
| name        | value         |
+-------------+---------------+
| High        | 45            |
| Description | Rain And Snow |
| Low         | 25            |
| Day         | Fri           |
| Date        | 29 Dec 2017   |
+-------------+---------------+
5 rows in set (0.00 sec)

我几乎通过以下查询实现了这一目标:

select

  if (rfpv.name = 'Date', rfpv.value, null) as `Date`,
  if (rfpv.name = 'High', rfpv.value, null) as `High`,
  if (rfpv.name = 'Low', rfpv.value, null) as `Low`,
  if (rfpv.name = 'Description', rfpv.value, null) as `Description`

from rss_feed rf
join rss_feed_definition rfd on rf.rss_feed_definition_id = rfd.id
join rss_feed_property_value rfpv on rfpv.rss_feed_id = rf.id

where rfd.type = 'weather'
and rf.id = 31
order by rf.id;

产生以下输出:

+-------------+------+------+---------------+
| Date        | High | Low  | Description   |
+-------------+------+------+---------------+
| NULL        | 45   | NULL | NULL          |
| NULL        | NULL | NULL | Rain And Snow |
| NULL        | NULL | 25   | NULL          |
| NULL        | NULL | NULL | NULL          |
| 29 Dec 2017 | NULL | NULL | NULL          |
+-------------+------+------+---------------+
5 rows in set (0.00 sec)

我现在如何通过忽略空值只留下一行来使上面的内容变平，如下所示:

+-------------+------+------+---------------+
| Date        | High | Low  | Description   |
+-------------+------+------+---------------+
| 29 Dec 2017 | 45   | 25   | Rain and Snow |
+-------------+------+------+---------------+

Answer 1

通过 rf.id 聚合并使用条件聚合。我会在这里使用 CASE 表达式，因为与 MySQL 的 IF 相比，这种语法在数据库中得到更广泛的支持。

SELECT
    rf.id,
    MAX(CASE WHEN rfpv.name = 'Date'        THEN rfpv.value END) AS Date,
    MAX(CASE WHEN rfpv.name = 'High'        THEN rfpv.value END) AS High,
    MAX(CASE WHEN rfpv.name = 'Low'         THEN rfpv.value END) AS Low,
    MAX(CASE WHEN rfpv.name = 'Description' THEN rfpv.value END) AS Description
FROM rss_feed rf
INNER JOIN rss_feed_definition rfd
    ON rf.rss_feed_definition_id = rfd.id
INNER JOIN rss_feed_property_value rfpv
    ON rfpv.rss_feed_id = rf.id
WHERE
   rfd.type = 'weather' AND
   rf.id = 31
GROUP BY
    rf.id
ORDER BY
    rf.id;