df = pd.DataFrame({'SKU':['A','A','A','B','B','B','C','C'],
                   'time':['2021-1-1','2021-1-3','2021-1-2','2021-1-2','2021-1-4','2021-1-3','2021-1-8','2021-1-6'],
                   'value':[1,2,3,4,5,6,7,8]})
df.set_index(['SKU', 'time'],inplace=True)

data = df.sort_index(level=["SKU","time"]).reset_index(['SKU','time']).groupby('SKU').apply(lambda x:x.iloc[0,:])
data.set_index('SKU')