The aim of this study is to investigate the efficacy, safety and tolerability of BI 10773 compared to placebo and sitagliptin given for 24 weeks as monotherapy in patients with T2DM with insufficient glycaemic control. For the open-label part of the study the objective is to estimate the efficacy and safety of BI 10773 when given for 24 weeks in patients with T2DM with very poor glycaemic control.