In-depth use of IsNull and nullif in SQL Server _mssql

ISNULL in SQL Server differs from ISNULL in ASP, with two parameters in SQL Server, syntax:
ISNULL (Check_expression, Replacement_value)

check_expression and Replacement_value data types must be consistent
• If Check_expression is NULL, return Replacement_value
• If check_expression is not NULL, return check_expression

Nullif is used to check two expressions, syntax:
Nullif (expression, expression)

• If two expression are equal, NULL is returned, which is the data type of the first expression
• If two expression are not equal, return the first Expressio