Vous y êtes presque avec votre requête. Utilisez simplement le filter clause pour obtenir le nombre de votes de l'utilisateur actuel
SELECT
post.postID as postID,
post.title as title,
post.author as author,
post.created as created,
COALESCE(sum(votes.vote), 0) as voteCount,
COALESCE(sum(votes.vote) filter (where votes.username= 'username'), 0) as userVote -- in '' just provide username for current login user
FROM post
LEFT JOIN votes ON post.postID = votes.postID
GROUP BY 1,2,3,4
ORDER BY 5 DESC
Une autre méthode consiste à utiliser une autre jointure à gauche comme ci-dessous :
SELECT
p.postID as postID,
p.title as title,
p.author as author,
p.created as created,
COALESCE(sum(v1.vote), 0) as voteCount,
COALESCE(v2.vote , 0) as userVote -- in '' just provide username for current login user
FROM post p
LEFT JOIN votes v1 ON p.postID = v1.postID
LEFT JOIN votes v2 on p.postID = v2.postID and v2.username='username'
GROUP BY 1,2,3,4,v2.vote
ORDER BY 5 DESC
