Nisarg Upadhyay shows three ways to calculate the median in T-SQL:

To calculate the median of any dataset, we first need to arrange all values from the dataset in a specific order. After arranging the data, we must determine the middle value of the specified dataset. If the dataset contains an odd number of values, than the middle value of the entire dataset will be considered as a median. Following is the example:

Median (M) = value of ((X + 1)/2)^{th}item. (x is the number of values in the dataset)

Honestly, CLR’s probably the best approach here if you want a fast calculation for a reasonably large number of rows. Using ML Services and R/Python is another alternative, though the launchpad spinup time will probably make it slower than CLR.