SELECT
Id, A, B, C,
MinVal
FROM #MinAmt
CROSS APPLY (
SELECT MIN(a)
FROM (VALUES (A),(B),(C) ) v (a)
WHERE a > 0
) x (MinVal)
😉
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden