mysql中BIT_COUNT的统计使用

3.6 . 8 . 根据天计算访问量

下面的例子显示了如何使用位组函数来计算每个月中用户访问网页的天数。

CREATE   TABLE  t1 ( year   YEAR ( 4 ),  month   INT ( 2 ) UNSIGNED ZEROFILL,

             
day   INT ( 2 ) UNSIGNED ZEROFILL);

INSERT   INTO  t1  VALUES ( 2000 , 1 , 1 ),( 2000 , 1 , 20 ),( 2000 , 1 , 30 ),( 2000 , 2 , 2 ),

            (
2000 , 2 , 23 ),( 2000 , 2 , 23 );

示例表中含有代表用户访问网页的年-月-日值。可以使用以下查询来确定每个月的访问天数:

SELECT   year , month ,BIT_COUNT(BIT_OR( 1 << day ))  AS  days  FROM  t1

       
GROUP   BY   year , month ;

将返回:

+ -- ----+-------+------+

|   year   |   month   |  days  |

+ -- ----+-------+------+

|   2000   |      01   |      3   |

|   2000   |      02   |      2   |

+ -- ----+-------+------+

该查询计算了在表中按年
/ 月组合的不同天数,可以自动去除重复的询问。
上面例子中的查询语句:

SELECT year,month,BIT_COUNT(BIT_OR(1<<day)) AS days FROM t1 GROUP BY year,month;

 


中的”BIT_COUNT(BIT_OR(1<<day)) 

“用法比较有技巧。

1、BIT_COUNT( expr ):返回 expr 的二进制表达式中”1“的个数。
        例如:29 = 11101 则:BIT_COUNT(29)= 4;
2、BIT_OR( expr ):返回 expr 中所有比特的bitwise OR。计算执行的精确度为64比特(BIGINT) 。
 
    例如:上面例子中,2000年02月中有一条2号的记录两条23号的记录,所以"1<<day"表示出来就是 “1<<2”和“1<<23”,得到二进制数 100 和 100000000000000000000000 。然后再OR运算。即 100 OR 10000000000000000000000 OR 10000000000000000000000 = 100000000000000000000100;这样再用BIT_COUNT处理得出的值就是2,自动去除了重复的日期。

别再手动算LLC了!用Mathcad 15分钟搞定谐振参数与增益曲线(附已验证产品案例) 本文详细介绍了如何利用Mathcad在15分钟内高效完成LLC谐振变换器的参数计算与增益曲线绘制。通过对比传统工具,突出Mathcad在自然数学表达、实时单位换算和动态参数关联方面的独特优势,帮助电源工程师大幅提升设计效率。文章还提供了已验证的产品案例和实用模板,助力工程师快速掌握谐振参数优化技巧。 阅读详情

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