The title says it all.
Is it possible to retrieve this information from a database command?
Here is a quick query in data explorer.
select sum(reputation) as total_rep from users
For our site, it returns 11803197 (as of Dec 2017)
This should be very easy to do with a simple SEDE query that just sums the reputation of all users. Using such a query (and considering the usual weekly update frequency of the data explorer) this reputation sum would be 6,317,380 at the time of answering.