mysql group_by

SELECT *,count(*) as c FROM .`bids_buy_info` AS o LEFT JOIN .`bids_users` AS u ON u.user_id=o.user_id left join .`bids_buy_goods` as bg on o.order_id = bg.order_id WHERE 1 AND o.bid_status != 5 group by o.order_id ORDER BY add_time DESC LIMIT 0,15"

这样的sql能查询到15个结果，去除group_by后如下，只查询到1个结果，为什么？

SELECT *,count(*) as c FROM `bids_buy_info` AS o LEFT JOIN `bids_users` AS u ON u.user_id=o.user_id left join `bids_buy_goods` as bg on o.order_id = bg.order_id WHERE 1 AND o.bid_status != 5 ORDER BY add_time DESC LIMIT 0,15

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没有group by，查询出来的是总数，当然只有一条了