Delete entries from releted tables

Question

I have got 2 tables: Contacts and Users.

Contacts table contains user_id which are referring to id in Users table. Contacts also contain list_type column.

Contacts: user_id, list_type Users: id, data

How can I delete entries/rows from both tables (Contacts and Users) that are referring to given list_type?

The trick is that i don't want to delete users who belongs to other contacts list_type.

EDIT:

Example:

Users (id,data)
1    John
2    Kate
3    Alan
4    Bob

Contacts (user_id, list_type)
1    1
3    1
1    2
4    2
2    2

Now I would like to delete list_type = 2, the result should be:

Users (id,data)
1    John - still is here, because it was also referring to list_type = 1 
3    Alan

Contacts (user_id, list_type)
1    1
3    1

Answer 1

from what i understood, i can give you this answer.. i guess users table has user_id field.. I gave this roughly, you make it comfortable to your code.. :D

   select user_ids from contacts where list_type=some_type;

    while(resultSet.next()){
         String userid=restultSet.getString(1);
         delete from users where user_ids=userid;
    }
    delete from contacts where list_type=some_type;

EDIT:

resultSet_1 = select user_ids from contacts where list_type=some_type;  

while(resultSet_1.next()){
   String userid=resultSet_1.getString(1);
   resultSet_2 = select count(user_ids) from contacts where user_id=userid group by user_ids;
   int count=resultSet_2.getInt(1);

   if(count==1){     //if more than 1 it means contact is in more than one list_type
       delete from users where user_ids=userid;
   }
}
delete from contacts where list_type=some_type;

Answer 2

Break it down into two steps;

SQLiteDatabase db;
        Cursor c = db
                .query("Contacts", new String[] { "user_id" }, "list_type=?",
                        new String[] { "list_type_1" }, null, null, null);

        if(c.moveToFirst()){
            do{
                db.delete("Users", "user_id=?",new String[]{c.getString(c.getColumnIndex("user_id"))});
            }while(c.moveToNext()));
        }

        db.delete("Contacts", "list_type=?", new String[]{"list_type_1"});

Answer 3

I would be tempted to do it with SQL. Let the database manage and clean itself, I say. Less back-and-forth between the database and your application, faster execution, and less code to maintain.

After any deletion from Contacts, run the following query:

DELETE FROM Users WHERE Id NOT IN (SELECT DISTINCT User_id FROM Contacts);

Or you could create a trigger and add it to your schema (which is even more hands-free!):

CREATE TRIGGER CleanUsers AFTER DELETE ON Contacts 
  BEGIN
    DELETE FROM Users WHERE Id NOT IN (SELECT DISTINCT User_id FROM Contacts);
  END;