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• If you use GROUP BY, output rows are sorted according to the GROUP BY columns as if you had an ORDER BY for the same columns. To avoid the overhead of sorting that GROUP BY produces, add ORDER BY NULL:

  SELECT a, COUNT(b) FROM test_table GROUP BY a ORDER BY NULL;

-- 载自 MySQL 手册.

上面说的是, group by 默认会排序, 如果不想这么做, 必须显式声明.

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