PL/SQL Table Function - How to return an empty result set? - oracle

Question

I'm trying to create a PL/SQL table function. I don't mind if it is PIPELINED or not. I just want it to return a query-able result set.
And I want to start with an empty result set. (Because it is possible that the result set I intend to construct will be empty.)
Is it possible to create a PL/SQL table function that returns zero rows?
In the books I have, and tutorials I can find, I only see examples that must return at least one record.
Example of the problem:
CREATE OR REPLACE PACKAGE z_util AS
TYPE t_row
IS RECORD (
CATEGORY VARCHAR2( 128 CHAR )
, MEASURE NUMBER
);
TYPE t_tab
IS TABLE OF t_row;
FUNCTION f_test
RETURN t_tab;
END z_util;
/
CREATE OR REPLACE PACKAGE BODY z_util AS
FUNCTION f_test
RETURN t_tab IS
retval t_tab;
BEGIN
RETURN retval;
END;
END z_util;
/
SELECT test.*
FROM TABLE ( z_util.f_test ) test;
Output:
Error starting at line : 24 in command -
SELECT test.*
FROM TABLE ( z_util.f_test ) test
Error at Command Line : 25 Column : 14
Error report -
SQL Error: ORA-00902: invalid datatype
00902. 00000 - "invalid datatype"
*Cause:
*Action:

Answer 1

Something like this?
SQL> CREATE TYPE t_row AS OBJECT (id NUMBER, name VARCHAR2 (50));
2 /
Type created.
SQL> CREATE TYPE t_tab IS TABLE OF t_row;
2 /
Type created.
SQL> CREATE OR REPLACE FUNCTION f_test
2 RETURN t_tab
3 AS
4 retval t_tab;
5 BEGIN
6 RETURN retval;
7 END;
8 /
Function created.
SQL> SELECT f_test FROM DUAL;
F_TEST(ID, NAME)
--------------------------------------------------------------------
SQL>
Saying that this is too simple and that it doesn't work while in package:
SQL> CREATE OR REPLACE PACKAGE pkg_test
2 AS
3 FUNCTION f_test
4 RETURN t_tab;
5 END;
6 /
Package created.
SQL> CREATE OR REPLACE PACKAGE BODY pkg_test
2 AS
3 FUNCTION f_test
4 RETURN t_tab
5 AS
6 retval t_tab;
7 BEGIN
8 RETURN retval;
9 END f_test;
10 END pkg_test;
11 /
Package body created.
SQL> select pkg_test.f_test from dual;
F_TEST(ID, NAME)
-------------------------------------------------------
SQL>
Works OK as well. Did you, by any chance, declare type within the package? If so, try to create it at SQL level.

Answer 2

See 6.4.6 Querying a Collection:
Note: In SQL contexts, you cannot use a function whose return type was declared in a package specification.
You cannot unnest the result of function, but the variable with the same data type:
create or replace package utils as
type t_row is record (category varchar2 (8), measure number);
type t_tab is table of t_row;
function passon (t t_tab:=null) return t_tab;
end utils;
/
create or replace package body utils as
function passon (t t_tab:=null) return t_tab is
begin
return t;
end;
end utils;
/
Usage and outcomes:
var rc refcursor
declare
tab1 utils.t_tab := utils.passon (); -- empty
tab2 utils.t_tab := utils.passon (utils.t_tab (utils.t_row ('category', 50)));
begin
open :rc for
select * from table (tab1) union all
select * from table (tab2);
end;
/
CATEGORY MEASURE
-------- ----------
category 50

Answer 3

jsut use a return statment without any parameters.
I have already one table function that splits a string based in token.
I modified the code for you as an example answet for your quetion.
If you pass the first string null, the function will return no rows, otherwise will return each token in separate row based on the second parameter offcurse.
// create needed types
// row object
CREATE OR REPLACE TYPE T_TOKEN_ROW AS OBJECT (
id NUMBER,
token_text VARCHAR2(50)
);
// table object
CREATE OR REPLACE TYPE T_TOKEN_TAB IS TABLE OF t_token_row;
// create table function to toknize a string
// input : P_string : the string to be toknized
// P_separator : a character to separate tokens
// Outputs : each token in separate record with id field
CREATE OR REPLACE FUNCTION PIPE_tokens (P_string varchar2,P_separator char) RETURN t_token_tab PIPELINED
AS
sLine VARCHAR2(2000);
nPos INTEGER;
nPosOld INTEGER;
nIndex INTEGER;
nLength INTEGER;
nCnt INTEGER;
sToken VARCHAR2(200);
BEGIN
if (P_string is null ) then
return ;
else
sLine := P_string;
IF (SUBSTR(sLine, LENGTH(sLine), 1) <> '|') THEN
sLine := sLine || '|';
END IF;
nPos := 0;
sToken := '';
nLength := LENGTH(sLine);
nCnt := 0;
FOR nIndex IN 1..nLength LOOP
IF ((SUBSTR(sLine, nIndex, 1) = P_separator) OR (nIndex = nLength)) THEN
nPosOld := nPos;
nPos := nIndex;
nCnt := nCnt + 1;
sToken := SUBSTR(sLine, nPosOld + 1, nPos - nPosOld - 1);
PIPE ROW(t_token_row(nCnt,sToken));
--tTokenTab(nCnt) := sToken;
END IF;
END LOOP;
RETURN;
end if;
END;
// 2 Test query
select * from table(PIPE_tokens(null,';')) ;
select * from table(PIPE_tokens('5;2;3',';')) ;