Add elements one at a time from PL/SQL variable into collection variable? - oracle

Question

I have a routine written in T-SQL for SQL Server. We are migrating to Oracle so I am trying to port it to PL/SQL. Here is the T-SQL routine (simplified); note the use of the table-valued variable which, in Oracle, will become a "nested table" type PL/SQL variable. The main thrust of my question is on the best ways of working with such "collection" objects within PL/SQL. Several operations in the ported code (second code sample, below) are quite awkward, where they seemed a lot easier in the SQL Server original:
DECLARE #MyValueCollection TABLE( value VARCHAR(4000) );
DECLARE #valueForThisRow VARCHAR(4000);
DECLARE #dataItem1Val INT, #dataItem2Val INT, #dataItem3Val INT, #dataItem4Val INT;
DECLARE theCursor CURSOR FAST_FORWARD FOR
SELECT DataItem1, DataItem2, DataItem3, DataItem4 FROM DataTable;
OPEN theCursor;
FETCH NEXT FROM theCursor INTO #dataItem1Val, #dataItem2Val, #dataItem3Val, #dataItem4Val;
WHILE ##FETCH_STATUS = 0
BEGIN
-- About 50 lines of logic that evaluates #dataItem1Val, #dataItem2Val, #dataItem3Val, #dataItem4Val and constructs #valueForThisRow
SET #valueForThisRow = 'whatever';
-- !!! This is the row that seems to have no natural Oracle equivalent
INSERT INTO #MyValueCollection VALUES(#valueForThisRow);
FETCH NEXT FROM theCursor INTO #dataItem1Val, #dataItem2Val, #dataItem3Val, #dataItem4Val;
END;
CLOSE theCursor;
DEALLOCATE theCursor;
-- !!! output all the results; this also seems harder than it needs to be in Oracle
SELECT * FROM #MyValueCollection;
I have been able to port pretty much everything, but in two places (see comments in the code), the logic is a lot more complex than the old SQL Server way, and I wonder if there might be, in Oracle, some more graceful way that is eluding me:
set serveroutput on; -- needed for DBMS_OUTPUT; see below
DECLARE
TYPE StringList IS TABLE OF VARCHAR2(4000);
myValueCollection StringList;
dummyTempCollection StringList; -- needed for my kludge; see below
valueForThisRow VARCHAR2(4000);
BEGIN
-- build all the sql statements
FOR c IN (
SELECT DataItem1, DataItem2, DataItem3, DataItem4 FROM DataTable;
)
LOOP
-- About 50 lines of logic that evaluates c.DataItem1, c.DataItem2, c.DataItem3, c.DataItem4 and constructs valueForThisRow
valueForThisRow := 'whatever';
-- This seems way harder than it should be; I would rather not need an extra dummy collection
SELECT valueForThisRow BULK COLLECT INTO dummyTempCollection FROM dual; -- overwrites content of dummy temp
myValueCollection := myValueCollection MULTISET UNION dummyTempCollection; -- merges into main collection
END LOOP;
-- output all the results... again, there's no shorter/easier/more-compact/single-line equivalent?
IF myValueCollection.COUNT > 0
THEN
FOR indx IN myValueCollection.FIRST .. myValueCollection.LAST
LOOP
DBMS_OUTPUT.PUT_LINE(myValueCollection(indx));
END LOOP;
END IF;
END;
/
Thanks in advance for any help!

Answer 1

Personally, I'd take the "50 lines of logic", move it into a function that you call in your SQL statement, and then do a simple BULK COLLECT to load the data into your local collection.
Assuming that you really want to load data element-by-element into the collection, you can simplify the code that loads the collection
DECLARE
TYPE StringList IS TABLE OF VARCHAR2(4000);
myValueCollection StringList := StringList();
valueForThisRow VARCHAR2(4000);
BEGIN
-- build all the sql statements
FOR c IN (
SELECT DataItem1, DataItem2, DataItem3, DataItem4 FROM DataTable;
)
LOOP
-- About 50 lines of logic that evaluates c.DataItem1, c.DataItem2, c.DataItem3, c.DataItem4 and constructs valueForThisRow
valueForThisRow := 'whatever';
myValueCollection.extend();
myValueCollection( myValueCollection.count ) := valueForThisRow;
END LOOP;
-- output all the results... again, there's no shorter/easier/more-compact/single-line equivalent?
IF myValueCollection.COUNT > 0
THEN
FOR indx IN myValueCollection.FIRST .. myValueCollection.LAST
LOOP
DBMS_OUTPUT.PUT_LINE(myValueCollection(indx));
END LOOP;
END IF;
END;
/
If you declare the collection as an associative array, you could avoid calling extend to increase the size of the collection. If you know the number of elements that you are going to load into the collection, you could pass that to a single extend call outside the loop. Potentially, you can also eliminate the valueForThisRow local variable and just operate on elements in the collection.
As for the code that processes the collection, what is it that you are really trying to do? It would be highly unusual for production code to write to dbms_output and expect that anyone will see the output during normal processing. That will influence the way that you would write that code. Assuming that your intention is really to just call dbms_output, knowing that will generally send the data into the ether
FOR indx IN 1 .. myValueCollection.count
LOOP
dbms_output.put_line( myValueCollection(indx) );
END LOOP;
This works when you have a dense collection (all indexes between 1 and the count of the collection exist and have values). If you might have a sparse collection, you would want to use FIRST, NEXT, and LAST in a loop but that's a bit more code.