Getting error while running function using T-SQL - sql

Question

I have created small table value function using T-SQL which takes one input parameter phone number as and returns area code from it. Function compiles successfully but when I run it I am getting error:
Msg 4121, Level 16, State 1, Line 1776
Cannot find either column "dbo" or the user-defined function or aggregate "dbo.getareacode1", or the name is ambiguous
Refer to my code:
create or alter function dbo.getareacode1 (#phoneno as nvarchar(20))
returns table
with schemabinding
as
return
(select top 1
#phoneno as PhoneNumber, value as AreaCode
from
string_split(#phoneno,'-')
);
select dbo.getareacode1( N'323-234-2111');

Answer 1

First off, the order of the rows returned from STRING_SPLIT is not guaranteed, so this function broken to begin with.
But the error is caused by trying to invoke a Table-Valued Function where a scalar is expected.
A TVF cannot be treated like a scalar function. So
create or alter function dbo.getareacode1 (#phoneno as nvarchar(20))
returns table
with schemabinding
as
return
(
select top 1 #phoneno as PhoneNumber, value as AreaCode
from string_split(#phoneno,'-')
);
go
select * from dbo.getareacode1( N'323-234-2111');
And if you want to call it from another query use CROSS APPLY, eg
with phoneNumbers as
(
select N'323-234-2111' phoneno
from sys.objects
)
select ac.*
from phoneNumbers p
cross apply dbo.getareacode1(p.phoneno) ac;

Answer 2

On the flip-side of what David said: for your function to work like you were expecting you will need a scalar user-defined function (udf). This function:
-- Create function
create or alter function dbo.getareacode1 (#phoneno as nvarchar(20))
returns nvarchar(20) with schemabinding as
begin
return
(
select top (1) AreaCode = ss.[value]
from string_split(#phoneno,'-') ss
);
end
GO
... will allow this query to work:
select dbo.getareacode1( N'323-234-2111');
All that said, I strongly advise against using scalar udfs. I'll include a a couple good links that explain why at the bottom of this post. Leaving your function as-is and using APPLY, as David demonstrated, is the way to go. Also, string_split is not required here. If phone numbers are always coming in this format: NNN-NNN-NNNN, you could just use SUBSTRING:
SUBSTRING(N'323-234-2111', 1, 3).
For countries with varible-length area codes in the format (area code(2 or more digits))-NNN-NNNN you could do this:
declare #phoneno nvarchar(30) = N'39-234-2111'; -- Using a variable for brevity
select substring(#phoneno,1, charindex('-',#phoneno)-1);
If you, for whatever reason, really need a function, then this is how I'd write it:
-- using the variable-length requirement as an example
create or alter function dbo.getareacode1_itvf(#phoneno as nvarchar(20))
returns table with schemabinding as
return (select areacode = substring(#phoneno,1, charindex('-',#phoneno)-1));
go
Great articles about Scalar UDFs and why iTVFs are better:
How to Make Scalar UDFs Run Faster -- Jeff Moden
Scalar functions, inlining, and performance -- Adam Machanic
TSQL Scalar functions are evil – Andy Irving -- Stackoerflow post