DPP4 and type 2 diabetes mellitus: PubMed, EMBASE, Cochrane Library, Clinicaltrials(clinicaltrials.gov), Web of Science, China National Knowledge Infrastructure, China Science and Technology Journal Database and Wanfang Databases will be systematically searched, and all randomized controlled trials comparing DPP-4 inhibitors with placebo or other hypoglycemic drugs to study cognitive impairment in type 2 diabetic patients will be included.