Android使用Sqlite出現缺少"_id"列問題解決方法

在Android使用Sqlite時，《Head First Android 開發》裏就提到，一定要有"_id"列，一般平常建表都會把 _id 列建上，但是就不一定會讀取出來。多次都沒出錯，所以沒覺得這有啥重要的。直到有一次對多表進行聯合查詢時，麻煩纔出現。

有2個表：

public static final String C_MONTHDETAIL = "create table monthdetail(_id integer primary key autoincrement," +
            "sId integer," +
            "mId integer," +
            "mPrice float," +
            "mDetail text)";

public static final String C_SHARENAME = "create table sharename(_id integer primary key autoincrement," +
            "sName text," +
            "sCode text)";

現在要讀出mPrice和sName，這兩個表sId和sharename的_id是一致的。

String rawsql="SELECT monthdetail.mPrice,monthdetail.mDetail,sharename.sName FROM monthdetail INNER JOIN sharename ON monthdetail.sId=sharename._id WHERE monthdetail.mId=?";
            cursor_dList=sqLiteDatabase.rawQuery(rawsql,new String[]{String.valueOf(dataID)});
            SimpleCursorAdapter simpleCursorAdapter=new SimpleCursorAdapter(MainActivity.this,R.layout.main_detaillist,cursor_dList,
                    new String[]{"sharename.sName","monthdetail.mPrice"},new int[]{R.id.main_sharename,R.id.main_mPrice},0);
            detailList.setAdapter(simpleCursorAdapter);

其中dataID是當需查詢的mId值。

結果Activity報錯：

   java.lang.IllegalArgumentException: column '_id' does not exist

百度了一下，發現正是因爲使用INNER JOIN聯合查詢時，表內不存在 _id 列所導致的。有些大神使用AS重新命名一列可以解決問題。於是修改了一下代碼：

String rawsql="SELECT monthdetail.mPrice,monthdetail.mDetail,sharename.sName,monthdetail._id as _id  FROM monthdetail INNER JOIN sharename ON monthdetail.sId=sharename._id WHERE monthdetail.mId=?";
            cursor_dList=sqLiteDatabase.rawQuery(rawsql,new String[]{String.valueOf(dataID)});
            SimpleCursorAdapter simpleCursorAdapter=new SimpleCursorAdapter(MainActivity.this,R.layout.main_detaillist,cursor_dList,
                    new String[]{"sharename.sName","monthdetail.mPrice"},new int[]{R.id.main_sharename,R.id.main_mPrice},0);
            detailList.setAdapter(simpleCursorAdapter);

把monthdetail裏的_id列讀取出來，使用 as _id，變爲_id 列，因爲其在本表也是無重複的，所以跟普通的_id 列屬性一致。重新編譯，ListView的detailList就可以正常顯示了。