MYSQL：条件基于相同的表，不同列中的相同值

Question

I have a table administ (administrations) containing 4 columns adm_nr , pres_name (president name), year and vice_pres (vice president name). I need to select the vice_pres and the amount of times they are featured in the table.

Here is my SQL

SELECT vice_pres, COUNT(vice_pres)
FROM administ
GROUP BY vice_pres

The problem is that there is an extra condition, being I only want to see the results (names & count) for vice presidents who've also been president. So there is a value comparison between two columns of the same table, in which the value for vice_pres (the name of the vice president), needs to be equal to at least one value for pres_name in another row but within the same table (in that he's been president at some point).

I'm thinking there's going to be an self join situation but can't for the life of me figure out how to pen down this condition.

Thanks in advance!

Answer 1

Join table with itself to select only those who was president

SELECT t.vice_pres, COUNT(t.vice_pres)
FROM administ t join administ p on t.vice_pres=p.pres_name
GROUP BY t.vice_pres