mysql中统计当月每天的数据数量，没有数据，那天的数量赋值为0

可以使用以下SQL语句来获取当月每天的数据数量，并将没有数据的日期数量赋值为0：

SELECT DATE(date_column) AS date, COUNT(*) AS count
FROM (
  SELECT DATE_FORMAT(CURRENT_DATE(), '%Y-%m-%d') - INTERVAL (a.a + (10 * b.a) + (100 * c.a)) DAY AS date
  FROM (
    SELECT 0 AS a UNION ALL SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3 UNION ALL SELECT 4 UNION ALL SELECT 5 UNION ALL SELECT 6 UNION ALL SELECT 7 UNION ALL SELECT 8 UNION ALL SELECT 9
  ) AS a
  CROSS JOIN (
    SELECT 0 AS a UNION ALL SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3 UNION ALL SELECT 4 UNION ALL SELECT 5 UNION ALL SELECT 6 UNION ALL SELECT 7 UNION ALL SELECT 8 UNION ALL SELECT 9
  ) AS b
  CROSS JOIN (
    SELECT 0 AS a UNION ALL SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3 UNION ALL SELECT 4 UNION ALL SELECT 5 UNION ALL SELECT 6 UNION ALL SELECT 7 UNION ALL SELECT 8 UNION ALL SELECT 9
  ) AS c
) AS dates
LEFT JOIN table ON DATE(date_column) = dates.date
WHERE MONTH(dates.date) = MONTH(CURRENT_DATE())
GROUP BY dates.date
ORDER BY dates.date;

其中，`date_column`是包含日期的列名，`table`是表名。这条SQL语句会把当月的每一天的数量分别统计出来，并将没有数据的日期数量赋值为0。