select field value from table in android

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Mobile

Android

Hi I am working with database in android i am able to insert in Table but when i am going for check username and password from table the i am using cursor for return query value thet time my where caluse is notworking i am not getting field value.
Cursor cursor=new Cursor
cur=db.rawquery("select * from tablename where username='"+username variabe +"'",null);

this query allways return all field and i want to return only single row

thank you