Mysql序列(八)—— group by排序问题

本文介绍了MySQL中GROUP BY默认排序行为及如何通过ORDER BY NULL避免不必要的结果排序,同时解释了ORDER BY NULL对GROUP BY操作的影响。

摘要生成于 C知道 ,由 DeepSeek-R1 满血版支持, 前往体验 >

在mysql中,group by默认会执行排序:

By default, MySQL sorts GROUP BY col1, col2, ... queries as if you also included ORDER BY col1, col2, ... in the query. If you include an explicit ORDER BY clause that contains the same column list, MySQL optimizes it away without any speed penalty, although the sorting still occurs.

上述是mysql官方手册描述:

默认情况下,mysql会像order by col1, col2,....一样排序group by col1, col2,...查询。如果查询中明确包含了与group by同样列的order by子句,mysql将不做任何加速优化。

If a query includes GROUP BY but you want to avoid the overhead of sorting the result, you can suppress sorting by specifying ORDER BY NULL. For example:

INSERT INTO foo
SELECT a, COUNT(*) FROM bar GROUP BY a ORDER BY NULL;

如果你对结果不想做任何group by排序,可以通过order by null抑制其进行排序。

虽然抑制了对结果的排序,但是优化器可能任然使用排序来实现group by分组操作,order by null只是抑制了对结果的排序。

转载于:https://www.cnblogs.com/lxyit/p/9448495.html

评论
添加红包

请填写红包祝福语或标题

红包个数最小为10个

红包金额最低5元

当前余额3.43前往充值 >
需支付:10.00
成就一亿技术人!
领取后你会自动成为博主和红包主的粉丝 规则
hope_wisdom
发出的红包
实付
使用余额支付
点击重新获取
扫码支付
钱包余额 0

抵扣说明:

1.余额是钱包充值的虚拟货币,按照1:1的比例进行支付金额的抵扣。
2.余额无法直接购买下载,可以购买VIP、付费专栏及课程。

余额充值