android - NullPointerException at the first line at getReadableDatabase -

i getting nullpointer exception @ getreadbledatabase

 public list<offer> getoffers(string offer_category)         {             db = this.getreadabledatabase();             string sql = "select * offer_db "+table_offer + " "+ offer_category+ " "+ offer_category;             cursor cur = db.rawquery(sql,null);             cur.movetofirst();        } 

here logcat data

caused by: java.lang.nullpointerexception @ android.database.sqlite.sqliteopenhelper.getdatabaselocked(sqliteopenhelper.java:224) @ android.database.sqlite.sqliteopenhelper.getreadabledatabase(sqliteopenhelper.java:188) @ com.example.shreyfrapp.databasehandler.getoffers(databasehandler.java:61) @ com.example.shreyfrapp.fragmenttab1.oncreate(fragmenttab1.java:39) @ android.app.fragment.performcreate(fragment.java:1678) @ android.app.fragmentmanagerimpl.movetostate(fragmentmanager.java:859) @ android.app.fragmentmanagerimpl.movetostate(fragmentmanager.java:1062) @ android.app.backstackrecord.run(backstackrecord.java:684) @ android.app.fragmentmanagerimpl.execpendingactions(fragmentmanager.java:1447) @ android.app.activity.performstart(activity.java:5240) @ android.app.activitythread.performlaunchactivity(activitythread.java:2168) @ android.app.activitythread.handlelaunchactivity(activitythread.java:2245) @ android.app.activitythread.access$800(activitythread.java:135) @ android.app.activitythread$h.handlemessage(activitythread.java:1196) @ android.os.handler.dispatchmessage(handler.java:102) @ android.os.looper.loop(looper.java:136) @ android.app.activitythread.main(activitythread.java:5021) @ java.lang.reflect.method.invokenative(native method) @ java.lang.reflect.method.invoke(method.java:515) @ com.android.internal.os.zygoteinit$methodandargscaller.run(zygoteinit.java:827) @ com.android.internal.os.zygoteinit.main(zygoteinit.java:643) @ dalvik.system.nativestart.main(native method)

give query in capital , try "=" in place of "is".why u moving cursor first in select query?