How do I count the unique value pairs in two columns of a table?

Question

I have a table (Table 2) that is populated with data from an imported .csv file. On table 1 I need to count the unique value pairs in two columns of Table 2.
Here's what I have:
Date                                        Person
7/10/2011                         A
7/12/2011                         W
7/12/2011                         X
7/12/2011                         X
7/12/2011                         X
7/12/2011                         Z
7/14/2011                         Z
7/15/2011                 X
7/16/2011                         Z
I'm focusing on person "X" and can easily count how many days that person shows up but what I want is to see on how many unique days that person shows up.
Here's the result I'm looking for (Person "X" shows up on 2 different days - 3 times on 7/12/2011 and once on 7/15/2011):
X                    2
I can't seem to find a function that allows me to do that. I also am not allowed to modify Table 2 so that leaves me to come up with a solution on Table 1.
Any ideas would be greatly appreciated.

Answer 1

Hi John,
Not being allowed to modify Table 2 is a minor inconvenience. Just copy (using a formula) the necessary two columns onto Table 1.
Yellow columns may be hidden. The procedure progresses from left to right. All formulas are entered into row 2 then filled down that column to the end of the table. The table must be as long as the list in column A of Table 2.
A2: =Table 2::A
Fill right to column B.
Fill both columns down as far as needed.
I've used actual Date and Time values in column A, formatted to show only the Date part, but the technique will work with text in these cells, provided all cells representing the same 'date' have exactly the same content.
C2: =A&B
This concatenates the contents of each row of columns A and B into a single text string.
D2: =COUNTIF($C$2:C2,C)
This counts the number of occurrences of the Date&Name string on the current row from the first regular cell in column C (C2) to the current cell.
E2: =IF(COUNTIF($B$2:B2,B)=1,MAX($E$1:E1)+1,"")
This constructs the index of first occurrences of each name, in the order they first occur. The index is used by LOOKUP in column F.
F2: =IF(ROW()-1>MAX(E),"",LOOKUP(ROW()-1,$E,$B))
This uses the index value created in E as a search-for value to extract a single copy of the names in column B. The result is a list of all distinct names in the list. Note that spelling differences will be counted as distinct names.The IF statement stops the listing when the last distinct name is extracted.
G2: =IF(LEN(F)>0,COUNTIFS($B,"="&F,$D,"=1"),"")
This counts the number of 'first occurrences of distinct Date & Name strings for each name on the list (ie. the number of distinct dates on which each name appears in the original list).
All of the functions used are described, with at least one example for each, in the iWork formulas and Functions User Guide. You can download the guide, and the Numbers '09 User Guide, via the Help menu in numbers.
Regards,
Barry