在Android中使用SQLite获取RowID

Question

I am creating an app to record assignments for different classes. Each class has it's own unique ID so the assignments listed for each class don't overlap into other classes. Here is a method I made to find the rowid for a certain class.

public int getIdFromClassName(String className){
    String query = "SELECT rowid FROM " + CLASSES_TABLE_NAME + " WHERE " + CLASSES_COLUMN_NAME + " = '" + className + "'";
    SQLiteDatabase db = this.getReadableDatabase();
    Cursor res = db.rawQuery(query, null);
    return res.getColumnIndex("id");
}

However, this always returns a value of -1.

Any thoughts on what to change to return the proper rowid value?

EDIT:

@Override
public void onCreate(SQLiteDatabase db) {
    // TODO Auto-generated method stub
    db.execSQL(
            "create table " + CLASSES_TABLE_NAME + " " +
                    "(id integer primary key, " + CLASSES_COLUMN_NAME + " text)"
    );
    db.execSQL(
            "create table " + ASSIGNMENTS_TABLE_NAME + " " +
                    "(id integer primary key, " + ASSIGNMENTS_COLUMN_NAME + " text, " + ASSIGNMENTS_COLUMN_TOTAL_POINTS
                    + " INTEGER, " + ASSIGNMENTS_COLUMN_CLASS_ID + " INTEGER)");

}

Answer 1

Try this query below to create a new column name rowID :

Cursor cursor= db.rawQuery("SELECT *,"+CLASSES_TABLE_NAME +".rowid AS rowID"+" FROM "+CLASSES_TABLE_NAME , null);

And after this query you can fetch the real rowid from the rowID column :

while (cursor.moveToNext()) {
        long chatRow=cursor.getLong(
                cursor.getColumnIndexOrThrow("rowID"));
}

Answer 2

The column returned by your query is called rowid , so you will not find a column called id .

Ensure that you use the same column name in the query and in the call to getColumnIndex .

And the index of this column is always zero; you also need to read the actual value from the column:

int colIndex = res.getColumnIndexOrThrow("rowid"); // = 0
if (res.moveToFirst()) {
    return res.getInt(colIndex);
} else {
    // not found
}

However, Android has an helper function that makes it much easier to read a single value:

public int getIdFromClassName(String className){
    String query = "SELECT rowid" +
                   " FROM " + CLASSES_TABLE_NAME +
                   " WHERE " + CLASSES_COLUMN_NAME + " = ?;";
    SQLiteDatabase db = this.getReadableDatabase();
    return DatabaseUtils.longForQuery(db, query, new String[]{ className });
}

Answer 3

res.getColumnIndex("id") is going to get you the column index of the column with the name "id". and since you are getting -1, it cant find it.. are you sure your id column is not "_id"?

To get this to work you should do something like this..

res.getLong(res.getColumnIndex("_id"));

Cursor.getColumnIndex()

Cursor.getLong()

(I would recommend you use getLong() rather than getInt() because SQLite database ID's can get larger than int's).

Answer 4

public int getIdFromClassName(String className) {
        String query = "SELECT rowid FROM " + CLASSES_TABLE_NAME + " WHERE "
                + CLASSES_COLUMN_NAME + " = '" + className + "'";
        SQLiteDatabase db = this.getReadableDatabase();
        Cursor res = db.rawQuery(query, null);
        if (res != null) {
            if (res.moveToFirst()) {
                do {
                    return Integer.parseInt(res.getString(res.getColumnIndex("id"))); // if your column name is rowid then replace id with rowid
                } while (res.moveToNext());
            }
        } else {
            Toast.makeText(context, "cursor is null", Toast.LENGTH_LONG).show();
        }
    }

Check your Cursor if its null check your Log that your table is created successfully and if its not null make sure your table has column id in it.

Answer 5

You can try this:

public int getIdFromClassName(String className){
String query = "SELECT id FROM " + CLASSES_TABLE_NAME + " WHERE " + CLASSES_COLUMN_NAME + " = '" + className + "'";
SQLiteDatabase db = this.getReadableDatabase();
Cursor res = db.rawQuery(query, null);
int id=-1;
If(res!=null&&res.moveToFirst())
id=res.getInt(res.getColumnIndex("id"));
return id;
}