通过表a数据使用SQL实现不同月份的正式课和试听课数量(表b）？ 注：datetime为时间戳，lessontype = 1 为试听课，2为正式课

可以使用如下的SQL语句查询不同月份的正式课和试听课数量：

SELECT MONTH(a.datetime) AS month,
SUM(CASE WHEN a.lessontype = 1 THEN 1 ELSE 0 END) AS trial_lessons,
SUM(CASE WHEN a.lessontype = 2 THEN 1 ELSE 0 END) AS formal_lessons
FROM table_a AS a
JOIN table_b AS b ON a.lessonid = b.lessonid
GROUP BY MONTH(a.datetime)