SELECT  
    ISNULL(Nullif('abc', 'abc'), '123456') AS using_isnull,
    COALESCE(Nullif('abc', 'abc'), '123456') AS using_coalesce,
    ISNULL(Nullif('abc', 'abc'), 123456) AS int_using_isnull,
    COALESCE(Nullif('abc', 'abc'), 123456) AS int_using_coalesce

SELECT ISNULL(CAST(NULL AS INT), 5.5) -- Returns 5
SELECT COALESCE(CAST(NULL AS INT), 5.5) -- Returns 5.5

SELECT DATALENGTH(ISNULL(CAST(NULL AS VARCHAR(5)), N'Hello')) -- Returns 5
SELECT DATALENGTH(COALESCE(CAST(NULL AS VARCHAR(5)), N'Hello')) -- Returns 10

DECLARE @test TABLE (colA CHAR(3)) 

INSERT INTO @test (
	colA
)  
SELECT
    'abc'
UNION ALL
SELECT 
    NULL
    
SELECT  
    ISNULL(colA, '123456') AS using_isnull,
    COALESCE(colA, '123456') AS using_coalesce
 FROM
    @test