从相关表中删除条目

Question

我有 2 张桌子：Contacts 和 Users。

Contacts 表包含user_id，它们引用Users 表中的id。 Contacts 还包含list_type 列。

Contacts: user_id, list_type Users: id, data

如何从两个表（Contacts 和 Users）中删除引用给定 list_type 的条目/行？

诀窍是我不想删除属于其他联系人list_type的users。

编辑：

例子：

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

现在我想删除list_type = 2，结果应该是：

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

分为两步；

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 2

据我了解，我可以给你这个答案.. 我猜 users 表有 user_id 字段..我粗略地给出了这个，你让它对你的代码很舒服..：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;

编辑：

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 3

我很想用 SQL 来做。我说，让数据库自行管理和清理。数据库和应用程序之间的来回操作更少，执行速度更快，需要维护的代码更少。

从联系人中删除任何内容后，运行以下查询：

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

或者您可以创建一个触发器并将其添加到您的架构中（这更加免提！）：

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