how to select particular column data in SQLite db in android studio? - sqlite

Question

I want to take data from table name "transactions" in my SQLite db and return it to main activity in android studio.
This is my DatabaseHelper.java class code to return balance from transactions table:
public int previousBal(String activeuser){
SQLiteDatabase db=this.getReadableDatabase();
Cursor cursor=db.rawQuery("select balance from transactions where email=? order by t_id desc limit 1",new String[]{activeuser});
int balance = cursor.getInt(Integer.parseInt("balance"));
return balance;
}
This is the MainActivity code. I am using previousBal method to get data from the table:
String bal;
int previousBal=db.previousBal(activeuser);
bal= String.valueOf(previousBal);
System.out.println("PREVIOUS BALANCE IS======"+bal);
This code does not return anything. help me figure it out.

Answer 1

The issue you have, assuming that the active user string is a valid exisitng user, is that you are trying to extract from the position that is effectively before the first row.
That is, you need to MOVE to a row in the Cursor.
Additionally the Cursor's getInt method expects the offset of the column from which to get the data, which would be 0. However, it is better (less prone to error and generally more flexible) to use the Cursor's getColumnIndex method to retrieve the column's offset.
As such change :-
public int previousBal(String activeuser){
SQLiteDatabase db=this.getReadableDatabase();
Cursor cursor=db.rawQuery("select balance from transactions where email=? order by t_id desc limit 1",new String[]{activeuser});
int balance = cursor.getInt(Integer.parseInt("balance"));
return balance;
}
to :-
public int previousBal(String activeuser){
SQLiteDatabase db=this.getReadableDatabase();
Cursor cursor=db.rawQuery("select balance from transactions where email=? order by t_id desc limit 1",new String[]{activeuser});
if (cursor.moveToFirst()) {
int balance = cursor.getInt(cursor.getColumnIndex("balance"));
} else {
balance = 0;
}
return balance;
}
This will move to the first row, if there is one, and extract the vale in the balance column of the cursor.
If there is no row that matches the query's selection criteria then the else clause will set the balance to 0.
If multiple rows are extracted the value from the first row will be returned.
Addtional
Frequently the use of the SQLiteDatabase rawQuery is frowned upon unless it is necessary. In you case the SQLitedatabase query method can be used. As such, the recommended previousBal method would be :-
public int previousBal(String activeuser){
SQLiteDatabase db=this.getReadableDatabase();
Cursor.query(
"transactions", // name of the table to query
new String[]{"balanace"}, // String array of columns to extract
"email=?", // WHERE clause (? indicates an arg)
new String[]{activeuser}, // The list of args to replace the ? (or ?'s on a sequential basis)
null, // GROUP BY clause
null, // HAVING clause
"t_id DESC", // ORDER clause
"1" // LIMIT value as a String
);
if (cursor.moveToFirst()) {
int balance = cursor.getInt(cursor.getColumnIndex("balance"));
} else {
balance = 0;
}
return balance;
}
The query convenience method builds the SQL escaping characters and offers improved protection against SQL injection.