SQL——COALESCE()函数

最近在牛客网刷sql题的时候，认识了COALESCE()函数，打算做一个小记录。

题目如下：

一开始关于题解我做了一些记录，最初的想法是用UNION ALL，

SELECT DATE_FORMAT(submit_time, '%Y%m') AS submit_month,
       COUNT(question_id) AS month_q_cnt,
       ROUND(COUNT(question_id)/DAY(LAST_DAY(submit_time)), 3) AS avg_day_q_cnt
FROM practice_record
WHERE submit_time IS NOT NULL AND YEAR(submit_time)='2021'
GROUP BY DATE_FORMAT(submit_time, '%Y%m')

UNION ALL

SELECT '2021汇总' AS submit_month,
       COUNT(question_id) AS month_q_cnt,
       ROUND(COUNT(question_id)/31, 3) AS avg_day_q_cnt
FROM practice_record
WHERE submit_time IS NOT NULL AND YEAR(submit_time)='2021'
ORDER BY submit_month

提交通过，后面想尝试其他方法时查找资料发现还可以用COALESCE()函数。用法如下：

coalesce (expression_1, expression_2, ...,expression_n)依次参考各参数表达式，遇到非null值即停止并返回该值。

sql语句：

当avg_q_cnt为null值时，返回1，否则将返回avg_q_cnt的真实值。

当第一个参数avg_q_cnt不为null时，不论第二个参数avg_q_cnt1是否为null值，都只返回参数avg_q_cnt的真实值，当avg_q_cnt为null且avg_q_cnt1不为null时，返回参数avg_q_cnt1的真实值；只有当第一二个参数均为null值时，才返回1。

代码：

SELECT COALESCE(DATE_FORMAT(submit_time, '%Y%m'), '2021汇总') AS submit_month,
       COUNT(question_id) AS month_q_cnt,
       ROUND(COUNT(question_id)/MAX(DAY(LAST_DAY(submit_time))), 3) AS avg_day_q_cnt
FROM practice_record
WHERE submit_time IS NOT NULL AND YEAR(submit_time)='2021'
GROUP BY DATE_FORMAT(submit_time, '%Y%m') WITH ROLLUP

提交结果如下：

 此外，通过这道题还学到了另外一个知识点：WITH ROLLUP

with在sql语句中定义在group by之后，当需要对数据库进行分类统计的时候，往往会用上group by进行分组。而在group by后面还可以加入with cube或with rollup等关键字对数据进行汇总。不过这个cube在mysql中并不适用。

with rollup函数是对聚合函数进行求和，且是对group by后的第一个字段进行分组计算。