fix(issues): keep one row per issue when sorting by exclusive label order (#39364) (#39928)

Backport #39364 by @shoemoney

Sorting by an exclusive label scope joined `issue_label`, returning one
row per label. Issues with several labels were listed repeatedly and
pushed others off the page, and `IssueIDs` returned inflated totals.

The sort key now comes from a correlated subquery, so each issue yields
one row.

Fixes https://github.com/go-gitea/gitea/issues/35711

This base-branch update was prepared with AI assistance.

Co-authored-by: Jeremy Schoemaker <jeremy@shoemoney.com>
Co-authored-by: silverwind <me@silverwind.io>

models/issues/issue_search.go

@@ -75,11 +75,11 @@ func applySorts(sess db.Session, sortType string, priorityRepoID int64) {
 	// Since this sortType is dynamically created, it has to be treated specially.
 	if after, ok := strings.CutPrefix(sortType, ScopeSortPrefix); ok {
 		scope := after
-		sess.Join("LEFT", "issue_label", "issue.id = issue_label.issue_id")
-		// "exclusive_order=0" means "no order is set", so exclude it from the JOIN criteria and then "LEFT JOIN" result is also null
-		sess.Join("LEFT", "label", "label.id = issue_label.label_id AND label.exclusive_order <> 0 AND label.name LIKE ?", scope+"/%")
-		// Use COALESCE to make sure we sort NULL last regardless of backend DB (2147483647 == max int)
-		sess.OrderBy("COALESCE(label.exclusive_order, 2147483647) ASC").Desc("issue.id")
+		// a subquery instead of a JOIN keeps one row per issue, "exclusive_order=0" means unset and COALESCE sorts it last on every DB
+		sess.OrderBy("COALESCE((SELECT MIN(label.exclusive_order) FROM issue_label"+
+			" INNER JOIN label ON label.id = issue_label.label_id"+
+			" WHERE issue_label.issue_id = issue.id AND label.exclusive_order <> 0 AND label.name LIKE ?), 2147483647) ASC", scope+"/%").
+			Desc("issue.id")
 		return
 	}