Thanks a lot Neil.
I considered an excel file (the attached file) with two sheets named "Ukraine" and "Istanbul" . When I use the names of the sheets, I can not get the expected output.
In[12]:= Import["C:\\Users\\agri-ghorbani\\Desktop\\data.xlsx"][[1]]
Out[12]= {{51.3, 44.3}, {29.2, 102.8}, {76.9, 66.7}}
In[13]:= Import["C:\\Users\\agri-ghorbani\\Desktop\\data.xlsx"][[
Ukraine]];
During evaluation of In[13]:= Part::pkspec1: The expression Ukraine cannot be used as a part specification.
In[14]:= Import["C:\\Users\\agri-ghorbani\\Desktop\\data.xlsx"][[2]]
Out[14]= {{45.2, 39.7, 38.9}, {79.8, 65.6, 65.4}, {109.8, 24.6, 35.4}}
In[15]:= Import["C:\\Users\\agri-ghorbani\\Desktop\\data.xlsx"][[
Istanbul]];
During evaluation of In[15]:= Part::pkspec1: The expression Istanbul cannot be used as a part specification.
Attachments: