How to count number of unique values of a field in a tab-delimited text file?

Question

I have a text file with a large amount of data which is tab delimited. I want to have a look at the data such that I can see the unique values in a column. For example,

Answer 1

awk -F '\t' '{ a[$1]++ } END { for (n in a) print n, a[n] } ' test.csv