SQL Server NULL Integer to Empty String using ISNULL

Question

My curiosity always gets the best of me and I've searched online for an explanation to this and came up with nothing (could be because I didn't use the right terms.)

Can someone please explain why SQL Server returns a value of zero (0) when the following is executed, instead of an empty string ('').

    DECLARE @I AS INT
    SET @I = NULL
    SELECT ISNULL(@I, '') -- 0

Answer 1

As declared here, the second argument to ISNULL is the replacement_value, which "must be of a type that is implicitly convertible to the type of check_expresssion." Implicitly converting '' to INT results in 0.

Answer 2

Because @I is declared as an INT, the empty string is implicitly CAST as an integer resulting in a ZERO.