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(mysql.info.gz) Calculating days

Info Catalog (mysql.info.gz) Searching on two keys (mysql.info.gz) Examples (mysql.info.gz) example-AUTO_INCREMENT
 
 3.6.8 Calculating Visits Per Day
 --------------------------------
 
 The following example shows how you can use the bit group functions to
 calculate the number of days per month a user has visited a Web page.
 
      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);
 
 The example table contains year-month-day values representing visits by
 users to the page. To determine how many different days in each month
 these visits occur, use this query:
 
      SELECT year,month,BIT_COUNT(BIT_OR(1<<day)) AS days FROM t1
             GROUP BY year,month;
 
 Which returns:
 
      +------+-------+------+
      | year | month | days |
      +------+-------+------+
      | 2000 |    01 |    3 |
      | 2000 |    02 |    2 |
      +------+-------+------+
 
 The query calculates how many different days appear in the table for
 each year/month combination, with automatic removal of duplicate
 entries.
 
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