docs: add user stats commands

Author: m0wer

backend/docs/useful_queries.md

@@ -1,6 +1,33 @@
 # Useful dabase queries
 
 ## Remove duplicate articles (sqlite)
+
 ```sql
 DELETE FROM article WHERE rowid not in (select min(rowid) from article group by url);
 ```
+
+## Get all read article titles from a user
+
+```sql
+select title from article join userarticlelink on article.id = userarticlelink.article_id join user on userarticlelink.user_id = user.id where user.id = 'USERID';
+```
+
+## User stats including how many days they have been active
+
+```sql
+select id, created_at, last_request, Cast ((
+    JulianDay(last_request) - JulianDay(created_at)
+) As Integer) from user order by last_request ;
+```
+
+## Get all feeds for a user
+
+```sql
+select title, url from feed join userfeedlink on feed.id = userfeedlink.feed_id join user on userfeedlink.user_id = user.id where user.id = 'USERID';
+```
+
+## Get all read articles for a user
+
+```sql
+select title, url from article join userarticlelink on article.id = userarticlelink.article_id join user on userarticlelink.user_id = user.id where user.id = 'USERID';
+```