MySql-BIT_COUNT()&BIT_OR()

MySql---BIT_COUNT()&BIT_OR()

完全转载：http://msh24.iteye.com/blog/1136449Java代码??

1. CREATE?TABLE?t1?(year?YEAR(4),?month?INT(2)?UNSIGNED?ZEROFILL,??
2. ?????????????day?INT(2)?UNSIGNED?ZEROFILL);??
3. INSERT?INTO?t1?VALUES(2000,1,1),(2000,1,20),(2000,1,30),(2000,2,2),??
4. ????????????(2000,2,3),(2000,2,3);??
5. 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：??
6. ??
7. SELECT?year,month,BIT_COUNT(BIT_OR(1<<day))?AS?days?FROM?t1??
8. ???????GROUP?BY?year,month;??
9. 将返回：??
10. ??
11. +------+-------+------+??
12. |?year?|?month?|?days?|??
13. +------+-------+------+??
14. |?2000?|????01?|????3?|??
15. |?2000?|????02?|????2?|??
16. +------+-------+------+??
17. 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。??

<<位运算 使1左移了day位。?
1<<2=0100? 1<<3=1000?
bit_or：相同值or运算后值一样，最后把不一样的值加起来。?
1000 or 1000 or 0100 = 1100?
bit_count：统计被制1的个数。?
bit_count(1100) = 2;?

说白了意思就是统计被制1的个数。?

这个统计还可以这样写：select year, month, count(distinct day) from t1 group by year, month;

查看更多 下一篇