T-SQL and NULL values.
IS [NOT] NULL (T-SQL)
Determines whether a specified expression is NULL.
expression IS [ NOT ] NULL
– https://learn.microsoft.com/en-us/sql/t-sql/queries/is-null-transact-sql?view=sql-server-ver16
ISNULL (T-SQL)
Replaces NULL with the specified replacement value.
ISNULL ( check_expression , replacement_value )
– https://learn.microsoft.com/en-us/sql/t-sql/functions/isnull-transact-sql?view=sql-server-ver16
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