Why is T-SQL ISNULL() truncating the string and COALESCE is not?

Question

Given the following:

SELECT ISNULL(\'XY\' + NULL, \'ABCDEFGHIJ\') -- Outputs ABC (Why?)
SELECT COALESCE(\'XY\' + NULL, \'ABCDEFGHIJ\') -- Outputs ABCDEFGHIJ
         


        

Answer 1

ISNULL() converts the replacement value to the type of the check expression. In this case, the type of the check expression is CHAR(2), so converting the replacement value truncates it (are you sure you're getting ABC and not just AB?).

From the Microsoft documentation:

replacement_value can be truncated if replacement_value is longer than check_expression.