SQL Server 2022

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CREATE FUNCTION dbo.Try1 (@i int)
RETURNS int
AS BEGIN
  RETURN IIF(@i = 0, 0, @i + dbo.Try1(@i - 1));
END;
CREATE FUNCTION dbo.Try2 (@i int)
RETURNS int
WITH SCHEMABINDING
AS BEGIN
  RETURN IIF(@i = 0, 0, @i + dbo.Try2(@i - 1));
END;
Msg 4121 Level 16 State 1 Line 5
Cannot find either column "dbo" or the user-defined function or aggregate "dbo.Try2", or the name is ambiguous.

CREATE OR ALTER FUNCTION dbo.Try3 (@i int)
RETURNS int
WITH SCHEMABINDING
AS BEGIN RETURN NULL; END;
ALTER FUNCTION dbo.Try3 (@i int)
RETURNS int
WITH SCHEMABINDING
AS BEGIN
  RETURN IIF(@i = 0, 0, @i + dbo.Try3(@i - 1));
END;
Msg 4512 Level 16 State 3 Line 5
Cannot schema bind function 'dbo.Try3' because name 'dbo.Try3' is invalid for schema binding. Names must be in two-part format and an object cannot reference itself.

CREATE OR ALTER FUNCTION dbo.Try4 (@i int)
RETURNS int
WITH SCHEMABINDING
AS BEGIN
  RETURN IIF(@i = 0, 0, @i + dbo.Try4(@i - 1));
END;
Msg 4121 Level 16 State 1 Line 5
Cannot find either column "dbo" or the user-defined function or aggregate "dbo.Try4", or the name is ambiguous.

CREATE OR ALTER FUNCTION dbo.Try5 (@i int)
RETURNS int
WITH SCHEMABINDING
AS BEGIN RETURN NULL; END;
CREATE OR ALTER FUNCTION dbo.Try5 (@i int)
RETURNS int
WITH SCHEMABINDING
AS BEGIN
  RETURN IIF(@i = 0, 0, @i + dbo.Try5(@i - 1));
END;
CREATE OR ALTER FUNCTION dbo.Try6 (@i int)
RETURNS int
AS BEGIN RETURN NULL; END;
ALTER FUNCTION dbo.Try6 (@i int)
RETURNS int
WITH SCHEMABINDING
AS BEGIN
  RETURN IIF(@i = 0, 0, @i + dbo.Try6(@i - 1));
END;
Msg 4512 Level 16 State 3 Line 5
Cannot schema bind function 'dbo.Try6' because name 'dbo.Try6' is invalid for schema binding. Names must be in two-part format and an object cannot reference itself.