using dynamic sql to create column for select statement - sql

Question

I'm writing a stored procedure for paginated results and this result can be ordered by certain values. I did have a switch case in a select statement but because it was trying to do an orderby on rownum it was very slow.
Now I am trying to use dyanmic sql to build the query outside the select but I don't know if what I am doing is possible.
Here is my SQL in Oracle SQL Developer:
create or replace PROCEDURE Sp_tsa_trainees_pagination (
schemeid IN INT,
searchval IN VARCHAR2,
pagesize IN INT DEFAULT 20,
currentpage IN INT DEFAULT 1,
--orderby IN VARCHAR2,
cursor_ OUT SYS_REFCURSOR)
AS
-- LOCAL VARIABLES
totalcount INT;
numberofpages INT;
startposition NUMBER;
endposition NUMBER;
orderby VARCHAR2(100) := 'surname asc' ;
dynamic_query VARCHAR(255) := 'row_number() over (order by t.SURNAME DESC, t.FORENAMES DESC) AS rnum';
BEGIN
-- Get total number of trainees in scheme
select COUNT(t.ORG_REGISTRATION_ID)
into totalcount FROM v_trainee t
where t.ORG_REGISTRATION_ID = schemeid
AND t.status = 'A' and LOWER(t.trainee_name) like '%' || LOWER(searchval) || '%';
-- calculate number of pages in the pagination by dividing total number of records by how many to display for each page
numberofpages := totalcount / pagesize;
-- get start position by multiplying number of records to display for each page by current page
startposition := pagesize *( currentpage-1);
-- add calculated start position by number of records to display to get end position
endposition := startposition + pagesize;
CASE orderby
WHEN 'surname desc' THEN dynamic_query := 'row_number() over (order by t.SURNAME DESC, t.FORENAMES DESC) AS rnum';
WHEN 'surname asc' THEN dynamic_query := 'row_number() over (order by t.SURNAME ASC, t.FORENAMES ASC) AS rnum';
END CASE;
OPEN cursor_ FOR
Select * from
(
SELECT
-- order by based on selection
dynamic_query rnum,
t.ORG_REGISTRATION_ID SearchId,
t.FORENAMES Forenames,
t.FORENAME Forename,
t.SURNAME Surname,
t.person_id PersonId,
t.trainee_name TraineeName,
t.STATUS Status,
t.IPD_ANNUAL_REVIEW_DATE AnnualReviewDate,
t.ANNUAL_REVIEW_STATUS AnnualReviewStatus,
t.payment_received PaymentRecieved,
t.TRAINEE_ID TraineeId,
t.IPD_SIGNUP_DATE IpdSignupDate,
t.START_DATE StartDate,
t.END_DATE EndDate,
t.LENGTH_ON_SCHEME LengthOnScheme,
t.EMPLOYEE_NUMBER EmploymentNumber,
t.SELECTED_LEVEL SelectedLevel,
t.SELECTED_LEVEL_DESCRIPTION SelectedLevelDescription,
t.ELIGIBLE_LEVEL EligibleLevel,
t.ELIGIBLE_LEVEL_DESCRIPTION EligibleLevelDescription,
sce.FORENAMES SceForenames,
sce.FORENAME SceForename,
sce.SURNAME SceSurname,
sce.mentor_name SceName,
sce.EMPLOYEE_NUMBER SceEmployeeNumber,
de.FORENAMES DeForenames,
de.FORENAME DeForename,
de.SURNAME DeSurname,
de.mentor_name DeName,
de.EMPLOYEE_NUMBER DeEmployeeNumber,
t.COMPLETED_ATTRIBUTE_LEVELS CompletedAttributeLevels,
t.ATTRIBUTE_LEVEL_COUNT AttributeLevelCount,
-- get percentage
CASE t.ATTRIBUTE_LEVEL_COUNT
WHEN 0 THEN 0
ELSE
COMPLETED_ATTRIBUTE_LEVELS / t.ATTRIBUTE_LEVEL_COUNT * 100
END percentage,
DECODE(F_ISTRAINEEGROUPMEMBER(t.ORG_REGISTRATION_ID, 'S', t.person_id),'Y','N','Y') WithoutTsaGroup,
orr.status SchemeStatus,
(select count(*) from TRAINING_GROUP_TRAINEE tgt where tgt.trainee_id = t.TRAINEE_ID) NUMBER_OF_GROUPS,
TotalCount
FROM v_trainee t
INNER JOIN org_registration orr ON t.ORG_REGISTRATION_ID = orr.id
LEFT OUTER JOIN v_mentor sce ON t.sce_id = sce.MENTOR_ID
LEFT OUTER JOIN v_mentor de ON t.de_id = de.MENTOR_ID
where t.ORG_REGISTRATION_ID = schemeid AND t.status = 'A'
and LOWER(t.trainee_name) like '%' || LOWER(searchval) || '%'
)
where rnum >= startposition and rnum <= endposition;
END;
I want to use this variable with the assigned sql:
dynamic_query rnum,
But when I execute the stored procedure I get this error:
ORA-01722: invalid number ORA-06512: at
"db.SP_TSA_TRAINEES_PAGINATION", line 46 ORA-06512: at line 13
So basically my question is can I assign a SQL to VARCHAR2 and then use it in a select statement dynamically.

Answer 1

You may need dynamic SQL for this. For example:
create or replace procedure testDyn(n in number, C OUT SYS_REFCURSOR) is
vDynamicPart varchar2(1000);
vSQl varchar2(1000);
begin
--
if (n = 1) then
vDynamicPart := 'count(1)';
else
vDynamicPart := 'count(null)';
end if;
--
vSQl := 'select ' || vDynamicPart || ' from dual';
open C for vSQl;
end;
If you call it
declare
n1 number;
n2 number;
C1 SYS_REFCURSOR;
C2 SYS_REFCURSOR;
begin
testDyn(1, C1);
testDyn(2, C2);
fetch C1 into n1;
fetch C2 into n2;
dbms_output.put_line('n1: ' || n1);
dbms_output.put_line('n2: ' || n2);
end;
you get:
n1: 1
n2: 0