[英]Obtaining a max row using EXISTS with additional condition/s
我在MySQL数据库中有一个表,即weight
。 可以在以下快照中查看它。
重量表:
:
我需要以下查询。
SELECT *
FROM weight w
WHERE w.weight_id IN (SELECT MAX(ww.weight_id)
FROM weight ww
WHERE ww.weight BETWEEN 0 AND 60);
从weight
表中从语言上获得最大行,例如weight
weight
列的值在0到60之间。
我想要使用EXISTS()
进行相同的查询。 以下尝试失败。
SELECT *
FROM weight w
WHERE NOT EXISTS (SELECT 1
FROM weight ww
WHERE w.weight_id < ww.weight_id
AND ww.weight BETWEEN 0 AND 60);
它返回从最大行开始的所有行(如果省略了BETWEEN
条件( AND ww.weight BETWEEN 0 AND 60
),那么它将返回表中所有行的最大值,但它不是所希望的。它应该返回一个最大行根据BETWEEN
条件应用过滤器后)。
稍作修改的版本将不返回任何行。
SELECT *
FROM weight w
WHERE EXISTS (SELECT 1
FROM weight ww
WHERE w.weight_id = ww.weight_id
AND ww.weight BETWEEN 0 AND 60
AND NOT EXISTS(SELECT 1
FROM weight www
WHERE ww.weight_id < www.weight_id))
我还能使用EXISTS()
吗?
我使用的是ORM框架,其中FROM
子句中不支持子查询。 因此,如果排除FROM
的子查询,那就更好了。
一般来说, id
应该仅用作行标识符,而不能使用MAX()
进行比较。
如果要使一行的weight
介于两个值之间:
SELECT id, weight
FROM weight
WHERE weight BETWEEN 0 AND 60
ORDER BY weight DESC
LIMIT 1
我不会在这种情况下使用EXISTS
。
更新
如果您不能使用LIMIT
则:
SELECT id, weight
FROM weight
WHERE weight = (
SELECT MAX(wi.weight)
FROM weight wi
WHERE wi.weight BETWEEN 0 AND 60
)
会返回相同的值,前提是MAX(weight)
上没有重复
如果您坚持使用EXISTS
则可以尝试:
SELECT *
FROM weight w
WHERE NOT EXISTS (
SELECT 1
FROM weight ww
WHERE w.weight < ww.weight
AND ww.weight BETWEEN 0 AND 60
)
AND w.weight BETWEEN 0 AND 60
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