起因

最近有个需求需要在sql中实现获取近半年的统计数据，本来以为挺简单的，不过这个项目数据基本没有，在此情况下还要实现获取近半年的数据就没办法简单group by了

实现

#如果每个月都有数据的话是比较简单的
SELECT 
    DATE_FORMAT(date, '%Y-%m') AS month, 
    COUNT(*) AS number_of_invoices
FROM 
    in_storeage_form
WHERE 
    date BETWEEN DATE_SUB(CURDATE(), INTERVAL 6 MONTH) AND CURDATE()
GROUP BY 
    month

不过这种方式可能会存在漏掉月份的情况

#如果要在补齐月份使用这种
SELECT 
    DATE_FORMAT(date, '%Y-%m') AS month, 
    COUNT(*) AS number_of_invoices
FROM 
    in_storeage_form
WHERE 
    date BETWEEN DATE_SUB(CURDATE(), INTERVAL 6 MONTH) AND CURDATE()
GROUP BY 
    month
		
UNION ALL

SELECT 
    m.date as month,
    0 AS number_of_invoices
FROM 
    ( SELECT DATE_FORMAT(CURDATE(), '%Y-%m') AS date
    UNION ALL
    SELECT DATE_FORMAT(DATE_SUB(CURDATE(), INTERVAL 1 MONTH), '%Y-%m') date
    UNION ALL
    SELECT DATE_FORMAT(DATE_SUB(CURDATE(), INTERVAL 2 MONTH), '%Y-%m') date
    UNION ALL
    SELECT DATE_FORMAT(DATE_SUB(CURDATE(), INTERVAL 3 MONTH), '%Y-%m') date
    UNION ALL
    SELECT DATE_FORMAT(DATE_SUB(CURDATE(), INTERVAL 4 MONTH), '%Y-%m') date
	UNION ALL
    SELECT DATE_FORMAT(DATE_SUB(CURDATE(), INTERVAL 5 MONTH), '%Y-%m') date	) m
where not exists(
		SELECT 
    		DATE_FORMAT(f.date, '%Y-%m') AS date
		FROM in_storeage_form f
		WHERE date BETWEEN DATE_SUB(CURDATE(), INTERVAL 6 MONTH) AND CURDATE()
			and  DATE_FORMAT(f.date, '%Y-%m') = m.date
		GROUP BY date
	) 
order by month desc

结论

先通过普通的计算有数据的月份，在获取近6个月到底是哪几个月，通过这几个月去跟有数据的几个月做一个筛选，获取到没数据的那几个月，再去将没数据的那几个月设为0就OK了