mySQL 最大组数
mySQL Largest number by group
我弄乱这段代码已经有一段时间了。首先，我将 SQL table 设置为 char 而不是 decimal，因为我不希望该数字始终显示十进制值，除非它具有十进制值（是否有另一种方法可以做到这一点？ ).我在我的代码中将其更改为十进制，但我相信这就是问题所在。我正在尝试为每个用户每个物种拉最大的鱼，但没有成功。
SELECT * FROM (SELECT * FROM entries ORDER BY CAST(weight AS DECIMAL(9,3)) DESC) tmp WHERE username = :user GROUP BY species
编辑
我已经将我的 table 更改为使用 decimal(10,3) 而不是 char 并且一直在尝试这个公式
SELECT * FROM (SELECT * FROM entries ORDER BY CAST(weight AS INT) DESC) tmp WHERE username = 'BooF' GROUP BY species
但出于某种原因 returns 这个
username location species date length weight timestamp id
BooF Muskellunge Lake Black Crappie 2014-08-31 9.125 0.37 2014-12-20 10:48:06 13
BooF Black Lake Largemouth Bass 2014-07-03 16.75 2.62 2014-12-20 10:49:00 2
BooF Muskellunge Lake Northern Pike 2014-08-31 32.75 6.86 2014-12-20 10:49:37 14
BooF Lake Bonaparte Rock Bass 2014-09-27 7 0.30 2014-12-20 10:50:50 57
BooF Lake Ozonia Smallmouth Bass 2014-08-15 13 1.19 2014-12-20 10:51:14 1
BooF Stark Falls Reservoir Walleye 2014-08-15 16 0.97 2014-12-20 10:51:37 49
BooF Lake Bonaparte Yellow Perch 2014-09-27 8.5 0.40 2014-12-20 10:52:01 56
这是我正在使用的table
username location species date length weight timestamp id
BooF Lake Ozonia Smallmouth Bass 2014-08-15 13.000 1.190 2014-12-20 10:51:14 1
BooF Black Lake Largemouth Bass 2014-07-03 16.750 2.620 2014-12-20 10:49:00 2
BooF Muskellunge Lake Largemouth Bass 2014-08-31 12.000 1.000 2014-08-31 22:04:42 7
BooF Muskellunge Lake Largemouth Bass 2014-08-31 16.000 2.000 2014-08-31 22:04:42 8
BooF Muskellunge Lake Largemouth Bass 2014-08-31 14.000 1.000 2014-08-31 22:04:42 9
BooF Muskellunge Lake Largemouth Bass 2014-08-31 16.000 2.000 2014-08-31 22:04:42 10
BooF Muskellunge Lake Largemouth Bass 2014-08-31 14.000 2.000 2014-08-31 22:04:42 11
BooF Muskellunge Lake Largemouth Bass 2014-08-31 16.000 2.000 2014-08-31 22:05:53 12
BooF Muskellunge Lake Black Crappie 2014-08-31 9.125 0.370 2014-12-20 10:48:06 13
BooF Muskellunge Lake Northern Pike 2014-08-31 32.750 6.860 2014-12-20 10:49:37 14
BooF Narrow Lake Northern Pike 2014-03-15 20.000 2.000 2014-09-01 11:08:21 15
BooF Narrow Lake Largemouth Bass 2014-03-15 14.000 1.000 2014-09-01 11:08:21 16
BooF Butterfield Lake Largemouth Bass 2014-05-26 19.000 3.000 2014-09-01 11:08:21 17
BooF Butterfield Lake Largemouth Bass 2014-05-26 17.000 2.000 2014-09-01 11:08:21 18
BooF Red Lake Northern Pike 2014-06-21 22.000 2.000 2014-09-01 11:08:21 19
BooF Black Lake Largemouth Bass 2014-07-03 15.000 2.000 2014-09-01 11:12:08 20
BooF Black Lake Largemouth Bass 2014-07-03 15.000 2.000 2014-09-01 11:12:08 21
BooF Black Lake Largemouth Bass 2014-07-02 17.000 2.000 2014-09-01 11:12:08 22
BooF Black Lake Largemouth Bass 2014-07-01 15.000 2.000 2014-09-01 11:12:08 23
BooF Black Lake Largemouth Bass 2014-07-01 15.000 2.000 2014-09-01 11:12:08 24
BooF Black Lake Largemouth Bass 2014-06-30 19.250 4.100 2014-12-20 10:53:17 25
BooF Black Lake Northern Pike 2014-06-29 26.750 3.940 2014-12-20 10:52:38 26
BooF Stark Falls Reservoir Walleye 2014-08-15 16.000 0.970 2014-12-20 10:51:37 49
BooF Lake Bonaparte Yellow Perch 2014-09-27 8.500 0.400 2014-12-20 10:52:01 56
BooF Lake Bonaparte Rock Bass 2014-09-27 7.000 0.300 2014-12-20 10:50:50 57
正如您在查询后看到的那样，它发布了重 2.62 磅的黑湖大口黑鲈，但根据我的数据 sheet 它显示最大的黑鲈为 4.1，所以这就是我被困的地方。
一般来说，子查询中的 ORDER BY 是没有意义的。 （它仅在与 FETCH FIRST/LIMIT/TOP 等结合使用时起作用）
解决方案是使用相关子查询为 "main query" 的当前行的用户名、位置、物种组合找到最重的鱼。如果是平局，将返回两行。
SELECT *
FROM entries e1
WHERE username = :user
AND CAST(weight AS DECIMAL(9,3)) = (select max(CAST(weight AS DECIMAL(9,3)))
from entries e2
where e1.username = e2.username
and e1.location = e2.location
and e1.species = e2.species)
请注意，使用 char 表示重量仍然是一个糟糕的选择，因为在比较值时您必须转换双方。在 table!
中返回十进制
我弄乱这段代码已经有一段时间了。首先，我将 SQL table 设置为 char 而不是 decimal，因为我不希望该数字始终显示十进制值，除非它具有十进制值（是否有另一种方法可以做到这一点？ ).我在我的代码中将其更改为十进制，但我相信这就是问题所在。我正在尝试为每个用户每个物种拉最大的鱼，但没有成功。
SELECT * FROM (SELECT * FROM entries ORDER BY CAST(weight AS DECIMAL(9,3)) DESC) tmp WHERE username = :user GROUP BY species
编辑
我已经将我的 table 更改为使用 decimal(10,3) 而不是 char 并且一直在尝试这个公式
SELECT * FROM (SELECT * FROM entries ORDER BY CAST(weight AS INT) DESC) tmp WHERE username = 'BooF' GROUP BY species
但出于某种原因 returns 这个
username location species date length weight timestamp id
BooF Muskellunge Lake Black Crappie 2014-08-31 9.125 0.37 2014-12-20 10:48:06 13
BooF Black Lake Largemouth Bass 2014-07-03 16.75 2.62 2014-12-20 10:49:00 2
BooF Muskellunge Lake Northern Pike 2014-08-31 32.75 6.86 2014-12-20 10:49:37 14
BooF Lake Bonaparte Rock Bass 2014-09-27 7 0.30 2014-12-20 10:50:50 57
BooF Lake Ozonia Smallmouth Bass 2014-08-15 13 1.19 2014-12-20 10:51:14 1
BooF Stark Falls Reservoir Walleye 2014-08-15 16 0.97 2014-12-20 10:51:37 49
BooF Lake Bonaparte Yellow Perch 2014-09-27 8.5 0.40 2014-12-20 10:52:01 56
这是我正在使用的table
username location species date length weight timestamp id
BooF Lake Ozonia Smallmouth Bass 2014-08-15 13.000 1.190 2014-12-20 10:51:14 1
BooF Black Lake Largemouth Bass 2014-07-03 16.750 2.620 2014-12-20 10:49:00 2
BooF Muskellunge Lake Largemouth Bass 2014-08-31 12.000 1.000 2014-08-31 22:04:42 7
BooF Muskellunge Lake Largemouth Bass 2014-08-31 16.000 2.000 2014-08-31 22:04:42 8
BooF Muskellunge Lake Largemouth Bass 2014-08-31 14.000 1.000 2014-08-31 22:04:42 9
BooF Muskellunge Lake Largemouth Bass 2014-08-31 16.000 2.000 2014-08-31 22:04:42 10
BooF Muskellunge Lake Largemouth Bass 2014-08-31 14.000 2.000 2014-08-31 22:04:42 11
BooF Muskellunge Lake Largemouth Bass 2014-08-31 16.000 2.000 2014-08-31 22:05:53 12
BooF Muskellunge Lake Black Crappie 2014-08-31 9.125 0.370 2014-12-20 10:48:06 13
BooF Muskellunge Lake Northern Pike 2014-08-31 32.750 6.860 2014-12-20 10:49:37 14
BooF Narrow Lake Northern Pike 2014-03-15 20.000 2.000 2014-09-01 11:08:21 15
BooF Narrow Lake Largemouth Bass 2014-03-15 14.000 1.000 2014-09-01 11:08:21 16
BooF Butterfield Lake Largemouth Bass 2014-05-26 19.000 3.000 2014-09-01 11:08:21 17
BooF Butterfield Lake Largemouth Bass 2014-05-26 17.000 2.000 2014-09-01 11:08:21 18
BooF Red Lake Northern Pike 2014-06-21 22.000 2.000 2014-09-01 11:08:21 19
BooF Black Lake Largemouth Bass 2014-07-03 15.000 2.000 2014-09-01 11:12:08 20
BooF Black Lake Largemouth Bass 2014-07-03 15.000 2.000 2014-09-01 11:12:08 21
BooF Black Lake Largemouth Bass 2014-07-02 17.000 2.000 2014-09-01 11:12:08 22
BooF Black Lake Largemouth Bass 2014-07-01 15.000 2.000 2014-09-01 11:12:08 23
BooF Black Lake Largemouth Bass 2014-07-01 15.000 2.000 2014-09-01 11:12:08 24
BooF Black Lake Largemouth Bass 2014-06-30 19.250 4.100 2014-12-20 10:53:17 25
BooF Black Lake Northern Pike 2014-06-29 26.750 3.940 2014-12-20 10:52:38 26
BooF Stark Falls Reservoir Walleye 2014-08-15 16.000 0.970 2014-12-20 10:51:37 49
BooF Lake Bonaparte Yellow Perch 2014-09-27 8.500 0.400 2014-12-20 10:52:01 56
BooF Lake Bonaparte Rock Bass 2014-09-27 7.000 0.300 2014-12-20 10:50:50 57
正如您在查询后看到的那样，它发布了重 2.62 磅的黑湖大口黑鲈，但根据我的数据 sheet 它显示最大的黑鲈为 4.1，所以这就是我被困的地方。
一般来说，子查询中的 ORDER BY 是没有意义的。 （它仅在与 FETCH FIRST/LIMIT/TOP 等结合使用时起作用）
解决方案是使用相关子查询为 "main query" 的当前行的用户名、位置、物种组合找到最重的鱼。如果是平局，将返回两行。
SELECT *
FROM entries e1
WHERE username = :user
AND CAST(weight AS DECIMAL(9,3)) = (select max(CAST(weight AS DECIMAL(9,3)))
from entries e2
where e1.username = e2.username
and e1.location = e2.location
and e1.species = e2.species)
请注意，使用 char 表示重量仍然是一个糟糕的选择，因为在比较值时您必须转换双方。在 table!
中返回十进制