如何根据另一列的值获取单行值?
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【中文标题】如何根据另一列的值获取单行值?【英文标题】:How to get single row of values based on value of another column? 【发布时间】:2018-06-10 13:30:20 【问题描述】: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点:
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 null 值以仅保留一行来展平上述内容,如下所示:
+-------------+------+------+---------------+
| Date | High | Low | Description |
+-------------+------+------+---------------+
| 29 Dec 2017 | 45 | 25 | Rain and Snow |
+-------------+------+------+---------------+
【问题讨论】:
【参考方案1】:按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;
【讨论】:
【参考方案2】:只需将MAX()
放在您想要折叠在一起的所有内容周围(MAX()
忽略NULL
值),然后将GROUP BY
放在您尝试将所有内容折叠到其中的 id 上。 ..
SELECT
rf.id,
MAX(if (rfpv.name = 'Date', rfpv.value, null)) AS `Date`,
MAX(if (rfpv.name = 'High', rfpv.value, null)) AS `High`,
MAX(if (rfpv.name = 'Low', rfpv.value, null)) AS `Low`,
MAX(if (rfpv.name = 'Description', rfpv.value, null)) AS `Description`
FROM
rss_feed AS rf
INNER JOIN
rss_feed_definition AS rfd
ON rf.rss_feed_definition_id = rfd.id
INNER JOIN
rss_feed_property_value AS rfpv
ON rfpv.rss_feed_id = rf.id
WHERE
rfd.type = 'weather'
AND rf.id = 31
GROUP BY
rf.id
ORDER BY
rf.id
;
我将rf.id
字段添加到SELECT
只是为了更清楚地说明正在做什么。 (因此还添加了关联的GROUP BY rf.id
)
【讨论】:
【参考方案3】:我通过以下查询实现了这一点:
select
rf.id as rssFeedId,
(select value
from rss_feed_property_value
where name = 'Date'
and rss_feed_id = rssFeedId) as `Date`,
(select value
from rss_feed_property_value
where name = 'High'
and rss_feed_id = rssFeedId) as `High`,
(select value
from rss_feed_property_value
where name = 'Low'
and rss_feed_id = rssFeedId) as `Low`,
(select value
from rss_feed_property_value
where name = 'Description'
and rss_feed_id = rssFeedId) 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
group by rf.id
order by rf.id;
我不确定这是否是最有效的方法。
【讨论】:
如果你走这样的路线,你不需要最后一个JOIN
到 rfpv
因为你的相关子查询正在为你做这件事,这反过来意味着你赢了不需要GROUP BY
。也就是说,单个 JOIN
和 GROUP BY
将比 4 个相关子查询快得多。以上是关于如何根据另一列的值获取单行值?的主要内容,如果未能解决你的问题,请参考以下文章