Stata is excellent in merging data.
Your thinking of "因为科学计数法而失败" may not be right.
Your should first run the following to make sure you don't have any duplictates of code_p in your master dataset (also you have to know whether there are missing values in code_p since in some cases missing values
will cause troubles in merging datasets)

duplicates report CODE_P     
*if there are duplicates, should merge data like this:

merge m:1 CODE_P using "your using file pathway", keep(match) nogen



Good luck

把ID转为字符型呢？

variable newcode does not uniquely identify observations in the master data
哈哈，stata已经告诉你了问题是什么了…
主数据文件那个变量不是非重复的，
尝试m:1或者m:m试试…good luck！

试试

请问你的问题解决了吗，我也遇到同样的问题

不是ID长度的问题，CH**LS的ID也是12位的，merge一点问题也没有，你看看code的格式是不是一致，是不是唯一，因为你用的是1：1