表:t_test
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id(int) cost(int) des Autoid(id)
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1 10 aaaa 1
1 15 bbbb 2
1 20 cccc 3
2 80 aaaa 4
2 100 bbbb 5
2 60 dddd 6
3 500 dddd 7
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取每一类id中cost最大的纪录
1