我已经搞砸了这段代码很长一段时间了。首先我将我的SQL表设置为char而不是十进制，因为我不希望该数字始终显示十进制值，除非它有一个十进制值（还有另一种方法吗？）。我在代码中将其更改为十进制，但我相信这就是问题所在。我试图为每个物种的每个用户提取最大的鱼，但它没有成功。
SELECT * FROM (SELECT * FROM entries ORDER BY CAST(weight AS DECIMAL(9,3)) DESC) tmp WHERE username = :user GROUP BY species
修改
我已经将我的表改为使用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
但由于某种原因，它会返回此
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
这是我正在使用的表格
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
正如你在查询后看到的那样，它为重量为2.62磅的黑色湖泊发布了一个大嘴鲈鱼，但是在我的数据表中它显示了4.1的最大低音，所以这就是我被困住的地方。
答案 0 :(得分：1)
一般来说，子查询中的ORDER BY没有任何意义。 （仅当与FETCH FIRST / LIMIT / TOP等结合使用时才会这样做。）
解决方案是使用相关的子查询来查找“主查询”的当前行的用户名，位置，物种组合中最重的鱼。如果是平局，则返回两行。
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，对于重量而言，char仍然是一个糟糕的选择，因为在比较值时必须投射双方。在表格中回到小数点！