MySQL之聚合

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_CONCAT
select group_concat(DISTINCT b order by b desc separator ':') FROM z group by a;