Does SQL Server read all of a COALESCE function even if the first argument is not NULL?

I'm using a T-SQL COALESCE function where the first argument will not be null on about 95% of the times it is ran. If the first argument is NULL, the second argument is quite a lengthy process: SELECT COALESCE(c.FirstName ,(SELECT TOP 1 b.FirstName FROM TableA a JOIN TableB b ON .....) ) If, for example, c.FirstName = 'John', would SQL Server still run the sub-query? I know with the VB.NET IIF() function, if the second argument is True, the code still reads the third argument (even though it won't be used).