如何解决嵌套嵌套? MySQL查询帮助
| 长话短说,我正在尝试获取网站的最新更新时间。 我网站的主要内容存储在四个表中。 每个表都有一个updated_at字段。 我可以将它们合并到一个嵌套查询中:SELECT (
SELECT updated_at FROM proposition ORDER BY updated_at DESC LIMIT 1
) as latest_proposition,(
SELECT updated_at FROM product ORDER BY updated_at DESC LIMIT 1
) as latest_product,(
SELECT updated_at FROM review ORDER BY updated_at DESC LIMIT 1
) as latest_review,(
SELECT updated_at FROM segment ORDER BY updated_at DESC LIMIT 1
) as latest_segment
..这给了我一些像..
-------------------+-------------------+-------------------+-------------------
latest_proposition |latest_product |latest_review |latest_segment
-------------------+-------------------+-------------------+-------------------
2011-05-25 14:45:30|2011-05-25 14:37:28|2011-05-12 09:20:19|2011-05-12 09:20:19
-------------------+-------------------+-------------------+-------------------
1 row in set (0.00 sec)
我的问题是-现在如何获得这些领域的最新信息?获取整个“网站上次更新”时间。
提前致谢!
解决方法
select
max(updated_at) as updated_at
from
(
SELECT max(updated_at) as updated_at FROM proposition
UNION
SELECT max(updated_at) FROM product
UNION
SELECT max(updated_at) FROM review
UNION
SELECT max(updated_at) FROM segment
) x
,在返回的字段上使用GREATEST()(如果希望通过NULL
值,则将它们通过COALESCE()传递之后)。
,SELECT updated_at
from
(
SELECT updated_at FROM proposition ORDER BY updated_at DESC LIMIT 1
union all
SELECT updated_at FROM product ORDER BY updated_at DESC LIMIT 1
union all
SELECT updated_at FROM review ORDER BY updated_at DESC LIMIT 1
union all
SELECT updated_at FROM segment ORDER BY updated_at DESC LIMIT 1
) x
order by updated_at limit 1
,SELECT MAX(updated_at)
FROM (SELECT MAX(updated_at) FROM proposition UNION ALL
SELECT MAX(updated_at) FROM product UNION ALL
SELECT MAX(updated_at) FROM review UNION ALL
SELECT MAX(updated_at) FROM segment) x
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