sql server - 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

Why are these statements returning different results?

Answer 1

According to Microsoft documentation, for function:

ISNULL(check_expression, replacement_value)

replacement_value must be of a type that is implicitly convertible to the type of check_expression. Note that type for 'xy'+NULL is VARCHAR(3). Because of this your string 'ABCDEFGHIJ' is cast to VARCHAR(3) and thus trimmed.

It sounds strange why it is not VARCHAR(2), but this is the way it is - one character longer than 'xy'. You can play with this SQLFiddle and see for yourself that type for 'xy'+NULL is the same as for expression CASE WHEN 1=2 THEN 'XYZ' ELSE NULL END, which is NULL but is implicitly compatible to VARCHAR(3).

It seems that for expression 'xy'+NULL perceived length can be computed as 'xy' string length (2) plus 1 for every NULL added. For example, type of 'xy'+NULL+NULL is VARCHAR(4), type for 'xy'+NULL+NULL+NULL is VARCHAR(5) and so on - check out this SQLFiddle. This is extremely weird, but that is how MS SQL Server 2008 and 2012 work.