在mysql中，我們有時需要查詢某個時間段內的數據，但是不一定每個月都有數據，如何查找沒有數據的月份呢？下面是一種方法：

SELECT a.year,a.month 
FROM (
SELECT YEAR(date) AS year, MONTH(date) AS month 
FROM table
WHERE date BETWEEN '開始時間' AND '結束時間'
GROUP BY YEAR(date), MONTH(date)
) AS a
LEFT JOIN (
SELECT YEAR(date) AS year, MONTH(date) AS month 
FROM table
WHERE date BETWEEN '開始時間' AND '結束時間'
GROUP BY YEAR(date), MONTH(date)
) AS b
ON a.year = b.year AND a.month = b.month
WHERE b.year IS NULL

代碼解釋：

先通過對時間段內數據去重，取出所有月份，然后做一個自連接，找出有數據的月份與所有月份的交集，最終通過判斷右表沒有數據來確定不存在數據的月份，從而實現查詢沒有數據的月份。