count(*)和count(1)的区别

来源:互联网 发布:三端口环行器 编辑:程序博客网 时间:2024/05/17 21:56

count(*)和count(1)的区别我很容易混淆,在此记录一下。

select count(*) from dual;
<img src="data:image/png;base64,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" alt="" />select count(1) from dual;
<img src="data:image/png;base64,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" alt="" />
从上面的查询来看,都是数表dual有多少条记录。有这两种写法是因为:
①<span style="font-family: Arial, Helvetica, sans-serif;">count(*) 会走完所有的列,效率会低一些。</span>
<span style="font-family: Arial, Helvetica, sans-serif;"> count(1)选择数据表的第一列作为计数标准,当某行的第一列为空时,他就不计。</span>

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