It is rather difficult to calculate the reputation exactly from [SEDE](https://meta.mathoverflow.net/tags/data-explorer/info)<sup>1</sup>, but the total score (i.e., the sum of the scores of all user's posts) can serve as a reasonable proxy.

Here is a query which looks at such users: [Deleted accounts above the given score and number of posts](https://data.stackexchange.com/mathoverflow/query/1470017/deleted-accounts-above-the-given-score-and-number-of-posts?minscore=500&minposts=50). Notice that you can change the parameters minscore and minposts, you can even [set them both to zero](https://data.stackexchange.com/mathoverflow/query/1470017/deleted-accounts-above-the-given-score-and-number-of-posts?minscore=0&minposts=0). (As you have already mentioned, once you have userid, you can find more about that user, such as the list of their posts: [Is there a way to check all questions/answers posted from a deleted account](https://meta.mathoverflow.net/q/3748). I will add that sometimes you might be able to find also a username that the user had before the deletion - the methods mentioned here work also for deleted users: [Recognizing identifiable users changing user names](https://meta.mathoverflow.net/q/4426).)

You can make a similar query which lists both existing and deleted users: [Users above the given score and number of posts](https://data.stackexchange.com/mathoverflow/query/1470018/users-above-the-given-score-and-number-of-posts?minscore=500&minposts=50). Of course, if we're making a query combining both deleted and existing users, we can include also some data from the table Users for the ones which are not deleted, like [in this query](https://data.stackexchange.com/mathoverflow/query/1470030/users-above-the-given-score-and-number-of-posts).

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Re: *I have no idea how to find the userid of someone I used to know but have disappeared*. I will also add that if you have a post or a comment from a deleted user, you can see the userid there. You will find `userxyz` - where xyz is the userid - in places where you see username for the accounts which are not deleted.

If you remember username and you want userid, you might simply try to search [where the username was mentioned on MO](https://www.google.com/search?q=quid+site:mathoverflow.net) - which might lead you to some of their posts or comments. Or you can search for [comments mentioning this username](https://meta.mathoverflow.net/q/3247) - and you might be lucky enough that you'll find a comment responding to a comment from the delete user. Examples of such queries are [here](https://data.stackexchange.com/mathoverflow/query/556789/comments-containing-given-keyword-with-text-and-author?Word=quid) and [here](https://data.stackexchange.com/mathoverflow/query/556789/comments-containing-given-keyword-with-text-and-author?Word=@quid). (I was a bit hesitant whether to include specific examples here - since if some user decided to deleted their account, I do not want to attract more attention to them. Since [this particular user was discussed on meta](https://meta.mathoverflow.net/q/2827), I guess that using them as an example is ok - but I can always edit the links away if this would is viewed as problematic for some reason.)

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<sup>1</sup>Some discussions related to calculating reputation from SEDE: [How to retrieve exact reputation on "legendary day"?](https://meta.stackexchange.com/q/300600), [Reputation History from Data Explorer](https://meta.stackexchange.com/q/144716), [A query, days hitting rep cap vs days a member (rep)](https://meta.stackexchange.com/q/234720).