比较难的sql面试题，令我比较郁闷！
一组通话记录（总共500万条）: ID 主叫号码 被叫号码 通话起始时间 通话结束时间 通话时长198290000 0215466546656 2007-02-01 09:49:53.000 2007-02-01 09:50:16.000 23
298290000 0215466546662007-02-01 09:50:29.000 2007-02-01 09:50:41.000 12
398290000 0215466546662007-02-01 09:50:58.000 2007-02-01 09:51:12.000 14
468290900 0755133329866 2007-02-01 10:04:31.000 2007-02-01 10:07:13.000 162
578290000 0755255708638 2007-02-01 10:48:26.000 2007-02-01 10:49:23.000 57
678290000 0755821119109 2007-02-01 10:49:39.000 2007-02-01 10:52:55.000 196
778290000 0357309283702007-02-01 11:30:45.000 2007-02-01 11:31:58.000 73
878290000 0871138889904 2007-02-01 11:33:47.000 2007-02-01 11:35:00.000 73
968290000 0357309283792007-02-01 11:52:20.000 2007-02-01 11:54:56.000 156
10 68290000 0298521811199 2007-02-01 12:44:45.000 2007-02-01 12:45:04.000 19
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