How to select the maximum value from multiple columns

Depending on what you're trying to achieve...

1) If you want to get max of all columns in all rows:

SELECT MAX(C123) AS MaxValue
FROM (
	SELECT C1 AS C123
	FROM @tmp
	UNION ALL
	SELECT C2 AS C123
	FROM @tmp
	UNION ALL
	SELECT C3 AS C123
	FROM @tmp
	) AS T

--returns 1000

2) If you want to get max value of each column (in a single row):

SELECT MAX(C1) AS C1Max, MAX(C2) AS C2Max, MAX(C3) AS C3Max
FROM @tmp

-- returns:
--200	1000	780

3) If you want to get max value of each column (in rows with column names):

SELECT ColName, MAX(ColValue) AS MaxValueOfCol
FROM (
	SELECT ColName, ColValue
	FROM (
		SELECT C1, C2, C3
		FROM @tmp
	) AS PVT
	UNPIVOT(ColValue FOR ColName IN([C1], [C2], [C3])) AS UNPVT
	) AS T
GROUP BY ColName
--returns:
--C1	200
--C2	1000
--C3	780

Note: replace @tmp with the name of table in your database.