Calculate Standard Deviation corresponding to a category in another column

I have a list of values in one column and another column stating which category the set of values belongs too.

For example: A1:A100 belongs to category A. A100:A500 belongs to category B, etc. etc.

Searching where the categories begin and end to find where the cell numbers are (to calculate standard deviation for a given set) is getting extremely time consuming because the data set is enormous.

Instead of entering the cell numbers to give in the formula, is there a way to say, calculate for these values BUT only for category A.
So replace the cell number with an alphabetical command. The columns with the values and categories are next to each other.

Please let me know if this post needs clarification, it's a bit strange to explain without someone looking at the actual data.

Thank you!