Import a given column from an excel file?

It seems that I can get the first column of excle data, say "test.xls", if I know the total length...

data = Table[{i}, {i, 6}];
Export["test.xls", data];    
Import["test.xls", {"Data",1,Range[len],1}]

But the problem is, If I don't know the total length, can I do the same thing? especially consider the data scale is quite large.

Answer

The answer you are looking for is here: Import a Spreadsheet.

Thus you can use:

Import["~/test.xls", {"Data", 1, All, 2}]

The syntax in your case is {"Data", # of sheet(s), # of row(s), # of column(s)}.