We do enjoy killing off orphans

src/main/java/com/gmail/nossr50/database/SQLDatabaseManager.java

@@ -151,6 +151,7 @@ public final class SQLDatabaseManager {
         checkDatabaseStructure(DatabaseUpdateType.INDEX);
         checkDatabaseStructure(DatabaseUpdateType.MOB_HEALTHBARS);
         checkDatabaseStructure(DatabaseUpdateType.PARTY_NAMES);
+        checkDatabaseStructure(DatabaseUpdateType.KILL_ORPHANS);
     }
 
     /**
@@ -573,6 +574,34 @@ public final class SQLDatabaseManager {
                 write("ALTER TABLE `" + tablePrefix + "users` DROP COLUMN `party` ;");
                 break;
 
+            case KILL_ORPHANS:
+                mcMMO.p.getLogger().info("Killing orphans");
+                write(
+                        "DELETE FROM " + tablePrefix + "experience " +
+                         "WHERE NOT EXISTS (SELECT * FROM " +
+                         tablePrefix + "users u WHERE " + 
+                         tablePrefix + "experience.user_id = u.id);" 
+                         );
+                write(
+                        "DELETE FROM " + tablePrefix + "huds " +
+                         "WHERE NOT EXISTS (SELECT * FROM " +
+                         tablePrefix + "users u WHERE " + 
+                         tablePrefix + "huds.user_id = u.id);" 
+                         );
+                write(
+                        "DELETE FROM " + tablePrefix + "cooldowns " +
+                         "WHERE NOT EXISTS (SELECT * FROM " +
+                         tablePrefix + "users u WHERE " + 
+                         tablePrefix + "cooldowns.user_id = u.id);" 
+                         );
+                write(
+                        "DELETE FROM " + tablePrefix + "skills " +
+                         "WHERE NOT EXISTS (SELECT * FROM " +
+                         tablePrefix + "users u WHERE " + 
+                         tablePrefix + "skills.user_id = u.id);" 
+                         );
+                break;
+
             default:
                 break;
         }

src/main/java/com/gmail/nossr50/datatypes/database/DatabaseUpdateType.java

@@ -5,5 +5,7 @@ public enum DatabaseUpdateType {
     BLAST_MINING,
     INDEX,
     MOB_HEALTHBARS,
-    PARTY_NAMES,
+    PARTY_NAMES, 
+    KILL_ORPHANS,
+    ;
 }