Solidity智能合约是没法返回json对象、字典、结构体的。

https://vomtom.at/how-to-return-a-mapping-in-solidity-and-web3/

解决方案：返回多个数组，然后在应用层再进行拼接。

返回数组的方法：https://blog.csdn.net/u013288190/article/details/108832503

// 一定要在文件开头引入这个
pragma experimental ABIEncoderV2;


// 查询某个学生的所有成绩
function select_scores_info(address studentId) public view returns(string[], int[]){
 
		TableFactory tf = TableFactory(0x1001);
		Table table = tf.openTable("stu_score");
 
		string memory stuIdStr = addressToString(studentId);
		Condition condition = table.newCondition();
		condition.EQ("student_id", stuIdStr);
 
		Entries entries = table.select(stuIdStr, condition);
 
        // 初始化数组大小
        string[] memory user_courses_list = new string[](uint256(entries.size()));
        int[] memory user_scores_list = new int[](uint256(entries.size()));
        
        // 给数组赋值
		for(int i=0; i<entries.size(); ++i) {
		    
            Entry entry = entries.get(i);
            // 添加数组元素
            user_courses_list[uint256(i)] = entry.getString("course_name");
            user_scores_list[uint256(i)] = entry.getInt("score");

        }

        return (user_courses_list, user_scores_list);
 
}