A riddle was printed in the Winnipeg (Manitoba) Tribune on July 11, 1914:
 
Q: What’s the difference between a king and a bartender?
A: That’s easy. A king reigns and a bartender pours!
   
“Why did the King take a second job as a bartender? When it reigns it pours” was posted on Reddit—Jokes on July 6, 2019.
 
   
11 July 1914, Winnipeg (Manitoba) Tribune, second sec., pg. 4, col. 5:   
SURE
“What’s the difference between a king and a bartender?”
“That’s easy. A king reigns and a bartender pours!”
   
3 January 1916, St. Joseph (MO) Gazette, pg. 3, col. 1:
Sure
From Philadelphia Ledger
“What’s the difference between a king and a bartender?”
“That’s easy. A king reigns and a bartender pours.”
 
Google Books
The Marines Magazine
Volume 3
1917
Pg. 60:
(Overheard in the mess hall of the barracks detachment at Port Royal.)
Corporal Bethers: “Card, what’s the difference between a king and a bartender?”
Corporal Card: “Give it up.”
Corporal Bethers: “Why, one reigns and the other pours, of course.”
     
Twitter
Justin Ware
@justin_ware
“When he reigns, he pours!” would be a great tagline for a movie called KING BARTENDER
12:46 AM - 2 May 2018
   
Twitter
Ron’s Puns Rated
@RonsPunsRated
Hey, @RonSexsmith, did you hear about the bartender who became king of his country? When he reigns, he pours!
9:55 AM - 19 Mar 2019
 
Reddit—Jokes
Posted by u/Djpress913 July 6, 2019
Why did the King take a second job as a bartender?
When it reigns it pours.