» Counting the Number of Unique Items Sold by Each Salesperson

Problem:

Columns A:B contain a list of items sold and the ID of the salesperson who sold each of them.
We want to count the number of different items sold by each salesperson listed in column D.

Solution:

Use the SUM, MMULT, IF, and TRANSPOSE functions as shown in the following Array formula:
{=SUM(($A$2:$A$13=D2)/(($A$2:$A$13<>D2)+MMULT(--(IF($A$2:$A$13=D2,$B$2:$B$13)=TRANSPOSE($B$2:$B$13)),--($A$2:$A$13=D2))))}