表数据：

查询展示效果：

{
	"class_no": [
		 {"student_name": "张三", "sex": "男", "age": "12", "class": "一班"}
		,{"student_name": "李四", "sex": "男", "age": "12", "class": "一班"}
		,{"student_name": "王五", "sex": "男", "age": "11", "class": "一班"}
		,{"student_name": "赵六", "sex": "女", "age": "13", "class": "一班"}
		,{"student_name": "钱七", "sex": "男", "age": "14", "class": "二班"}
		,{"student_name": "王八", "sex": "女", "age": "15", "class": "二班"}
	]
}

方法一（手动转换）：

SELECT CONCAT("{\"class_no\":[",GROUP_CONCAT(a.msg),"]}") FROM 
(
SELECT 
class_no
,
GROUP_CONCAT(
"{"
"\"student_name\":\"",student_name,"\",",
"\"sex\":\"",sex,"\",",
"\"age\":\"",age,"\",",
"\"class\":\"",class_no,"\"","}"
) msg
FROM student_info  GROUP BY class_no
)a

方法二（mysql自带的函数转换）：

SELECT CONCAT('[',GROUP_CONCAT(JSON_OBJECT( 
'student_name',student_name,'sex',sex,'age',age,'class_no',class_no)),']')FROM student

注意：以上SQL查询方法是使用了

GROUP_CONCAT()

1、该函数有长度限制，一般默认长度为1024，可以使用这篇博客（https://blog.csdn.net/qq_39706570/article/details/103999965）轻松修改长度范围。