Counting the rows of multiple distinct columns

Answer 1

How about:

SELECT COUNT(*)
FROM (SELECT DISTINCT a, b FROM MyTable)

For more information on why this can't be done in a simpler way (besides concatenating strings as noted in a different answer), you can refer to the this Google Answers post: Sql Distinct Count.

Answer 2

You could concatenate a and b together into 1 string like this (TSQL, hopefully something very similar in Sybase:

SELECT COUNT(DISTINCT(STR(a) + ',' + STR(b)))
FROM @YourTable