Question

我使用以下方法删除SQLite表中的行：

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当数据库的大小大于8行时，我需要删除前8行，所以：

public void deleteFirstRow()
{
    SQLiteDatabase db = this.getWritableDatabase();

    Cursor cursor = db.query("eventosUTN", null, null, null, null, null, null);

    if(cursor.moveToFirst()) {
        String rowId = cursor.getString(cursor.getColumnIndex("id"));
        db.delete("eventosUTN", "id" + "=?",  new String[]{rowId});
    }
    db.close();
}

Events的getRowCount方法是：

public void checkRowsSize(Events events, Context contex)
{  
    new Thread(new Runnable() { 
        public void run() {
        if(events.getRowCount()>8){
            while(events.getRowCount()>8){
                events.deleteFirstRow(contex);
            }
        }
    }
}).start();
}

但是当我在checkRowSize中设置while循环时，它将删除所有数据库。为什么这个？但是如果我设置：

public int getRowCount(){
    String query = "Select*FROM eventsUTN";
    SQLiteDatabase db = this.getWritableDatabase();
    Cursor cursor = db.rawQuery(query, null);
    return cursor.getCount();
}

每次创建活动时，它仅删除第一行，直到数据库的大小小于8行。因此，没有while循环就可以正常工作！

Answer 1

当表的大小大于8行时，您可以删除前8行，该语句如下：

delete from eventosUTN
where 
  (select count(*) from eventosUTN) > 8
  and 
  id in (select id from eventosUTN limit 8)

其中id是表的primary key。
但是您应该知道，表中的前8行并不总是您所得到的。
表格行是无序的。
因此，也许您应该使用ORDER BY子句来定义此顺序的逻辑，例如：

delete from eventosUTN
where 
  (select count(*) from eventosUTN) > 8
  and 
  id in (select id from eventosUTN order by id limit 8)

因此，您可以在Java代码中执行以下操作：

String where = "(select count(*) from eventosUTN) > 8 and id in (select id from eventosUTN order by id limit 8)";
db.delete("eventosUTN", where, null);

如果表的primary key是integer并已定义为AUTOINCREMENT，这意味着它一直在增加并且从未被重用，那么您也可以使用以下语句：

delete from eventosUTN 
where id < (select id from eventosUTN order by id limit 8, 1)

并使用Java代码：

String where = "id < (select id from eventosUTN order by id limit 8, 1)";
db.delete("eventosUTN", where, null);

从SQLite数据库中删除前8行

1 个答案: