Daniel Hutmacher is making a list and checking it twice:
If you’ve worked with reporting, you’ve probably come across the following problem. You have a list of values, say “A, B, C, D, K, L, M, N, R, S, T, U, Z” that you want to display in a more user-friendly, condensed manner, “A-D, K-N, R-U, Z”.
Today, we’re going to look at how you can accomplish this in T-SQL, and what this has to do with window functions and gaps and islands.
Read on for a really good demonstration of how powerful window functions and STRING_AGG() can be.