Returns the first nonnull expression among its arguments.
COALESCE ( expression [ ,...n ] )
If all arguments are NULL, COALESCE returns NULL.
ISNULL() accepts exactly two parameters. If you want to take the first non-NULL among more than two values, you will need to nest your ISNULL() statements.
COALESCE(), on the other hand, can take multiple inputs.
SQL Server views a column created by COALESCE() as nullable, whereas one using ISNULL() is not.
Many uses of Coalesce
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