楼主这种情况下不会报错是因为只有一个variable id
如果你做出两个 id1 和 id2
情况会大不一样
实际数据操作中很少有这么简单的merge

楼主 您的code有小问题哦
merge m:m id using employment
应该是merge m:m id using employ

楼主的操作有问题啊。看看Stata自己的解释吧：
“m:m specifies a many-to-many merge and is a bad idea. In an m:m merge, observations are matched
within equal values of the key variable(s), with the first observation being matched to the first; the
second, to the second; and so on. If the master and using have an unequal number of observations
within the group, then the last observation of the shorter group is used repeatedly to match with
subsequent observations of the longer group. Thus m:m merges are dependent on the current sort
order—something which should never happen.
Because m:m merges are such a bad idea, we are not going to show you an example. If you think
that you need an m:m merge, then you probably need to work with your data so that you can use a
1:m or m:1 merge.”

说的很清楚，merge m:m 是要避免使用的

我在进行merge的时候也出现了这个问题，用来合并的关键变量在两个文件中都有重复，成了多对多合并了，请问这种情况下一定要修改一下数据吗？

如果有重复值，删除重复值，然后进行1:1, 1:m, 或者m:1的merge。
如果真要进行多对多，那么就用joinby。
总而言之，看你的需求。

好的！谢谢！

好的！谢谢！

merge m:m 貌似找不到什么好用途，他的合并结果有的难以解释，有的丢掉了一些我们想要的。

可是我在论坛上看到有人是这么说的“joinby is used for the cartesian product or outer join.  Merge is more about 1 to 1, 1 to many, many to 1 match.  Try some data and you can see the differences.  joinby is not used often.”
附上链接：https://bbs.pinggu.org/thread-2949443-1-1.html

+1我也想问