Excel - Count unique values that match ID, optimized for 100,000+ cases
问题 in reference to the excel screen capture below, I'm looking for a formula solution that counts the number of unique values in Column B (Colour) for each ID number in Column A. I've imputed the desired result in Column C. So, for instance, ID 1 (A2) has only one unique colour, Grey (B2), which would return a 1 in C2. ID 2 has only one unique colour, yellow (B3, B4), and returns 1 in C3 and C4. ID 3, has two unique colours, Blue and Purple, thus returning a 2 in C5 through C8. Etc. Because this