ISNULL VS COALESCE

For example Production.Product table records

Class	Color	ProductNumber
cl1	Null	Null
cl2	Null	Null
Null	co3	Null
Null	co4	Null
Null	Null	pn5
cl6	co6	Null
cl7	co7	pn7

ISNULL - if the first value is null, it will pick second value, if not first value will be return. Only 2 arguments can pass. Replaces NULL with the specified replacement value.

ISNULL ( check_expression , replacement_value )

SELECT  ISNULL(Class, Color) AS FirstNotNull FROM Production.Product
cl1
cl2
co3
co4
null
cl6
pn7

Coalesce returns the first non-null column from more than one columns
COALESCE ( expression [ ,...n ] )

SELECT  COALESCE(Class, Color, ProductNumber) AS FirstNotNull FROM Production.Product
cl1
cl2
co3
co4
pn5
cl6
pn7