EP:巧妙地过滤掉重复的日期

example:使用bit_or()  bit_count()巧妙地过滤掉重复的日期: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);+------+-------+------+| year | month | day  |+------+-------+------+| 2000 |    01 |   01 || 2000 |    01 |   20 || 2000 |    01 |   30 || 2000 |    02 |   02 || 2000 |    02 |   23 || 2000 |    02 |   23 |+------+-------+------+#计算天数去除重复天数SELECT year, month, BIT_COUNT(BIT_OR(1 << day)) AS days FROM t1 GROUP BY year, month;+------+-------+------+| year | month | days |+------+-------+------+| 2000 |    01 |    3 || 2000 |    02 |    2 |+------+-------+------+*****-GROUP_CONCATselect group_concat(DISTINCT b order by b desc separator ':') FROM z group by a;