How can I populate in memory tables, in PLSQL? - oracle

Question

I don't know if it is the correct terminology but I call "in memory tables" to the objects created like this:
create type InMemReg is object (field1 varchar2(10), field2 varchar2(20), field3 number);
create type InMemTab is table of InMemReg;
In this case my "in memory table" is "InMemTab". My question is how can I populate this kind of object, when i don't know previously the numbers of elements? I have seen in some places this type of initialization:
declare
v_uno InMemReg := InMemReg('a','b',1999);
v_dos InMemReg := InMemReg('A','Z',2000);
t_tres InMemTab := InMemTab();
begin
t_tres := InMemTab(v_uno, v_dos);
In this situation I have explicitly 2 objects before initialize "t_tres", but in a dynamic scenario where I could have n numbers of elements I don't know how to populate it.
In another OO language could be something like this:
t_tres.add(OtherObject)

Answer 1

The type InMemTab is a nested table in Oracle parlance.
The equivalent to the add method would be to call the extend method and then to assign OtherObject to the last position in the nested table.
SQL> ed
Wrote file afiedt.buf
1 declare
2 v_uno InMemReg := InMemReg('a','b',1999);
3 v_dos InMemReg := InMemReg('A','Z',2000);
4 t_tres InMemTab := InMemTab();
5 begin
6 t_tres.extend;
7 t_tres( t_tres.count ) := v_uno;
8 t_tres.extend;
9 t_tres( t_tres.count ) := v_dos;
10 dbms_output.put_line( 't_tres has ' || t_tres.count || ' elements.' );
11* end;
12 /
t_tres has 2 elements.
PL/SQL procedure successfully completed.
You can factor that out into an add procedure as well
SQL> ed
Wrote file afiedt.buf
1 declare
2 v_uno InMemReg := InMemReg('a','b',1999);
3 v_dos InMemReg := InMemReg('A','Z',2000);
4 t_tres InMemTab := InMemTab();
5 procedure add( p_nt IN OUT InMemTab,
6 p_elem IN InMemReg )
7 as
8 begin
9 p_nt.extend;
10 p_nt( p_nt.count ) := p_elem;
11 end;
12 begin
13 add( t_tres, v_uno );
14 add( t_tres, v_dos );
15 dbms_output.put_line( 't_tres has ' || t_tres.count || ' elements.' );
16* end;
17 /
t_tres has 2 elements.
PL/SQL procedure successfully completed.

Answer 2

It is common to populate the collection from the data itself, meaning you are not explicitly adding sets of strings and numbers, you're pulling the data in from other tables. Because this is a common and natural thing to do with collections, Oracle made it easy via "BULK COLLECT INTO" clause in pl/sql. For example:
DECLARE
TYPE EmployeeSet IS TABLE OF employees%ROWTYPE;
underpaid EmployeeSet;
-- Holds set of rows from EMPLOYEES table.
CURSOR c1 IS SELECT first_name, last_name FROM employees;
TYPE NameSet IS TABLE OF c1%ROWTYPE;
some_names NameSet;
-- Holds set of partial rows from EMPLOYEES table.
BEGIN
-- With one query,
-- bring all relevant data into collection of records.
SELECT * BULK COLLECT INTO underpaid FROM employees
WHERE salary < 5000 ORDER BY salary DESC;
-- Process data by examining collection or passing it to
-- eparate procedure, instead of writing loop to FETCH each row.
DBMS_OUTPUT.PUT_LINE
(underpaid.COUNT || ' people make less than 5000.');
FOR i IN underpaid.FIRST .. underpaid.LAST
LOOP
DBMS_OUTPUT.PUT_LINE
(underpaid(i).last_name || ' makes ' || underpaid(i).salary);
END LOOP;
-- You can also bring in just some of the table columns.
-- Here you get the first and last names of 10 arbitrary employees.
SELECT first_name, last_name
BULK COLLECT INTO some_names
FROM employees
WHERE ROWNUM < 11;
FOR i IN some_names.FIRST .. some_names.LAST
LOOP
DBMS_OUTPUT.PUT_LINE
('Employee = ' || some_names(i).first_name
|| ' ' || some_names(i).last_name);
END LOOP;
END;
/
You don't typically need to worry about extending or how many elements you'll have, you can usually slurp it in and then use the built in features of the collection as you like (counts, loop through, compare different collections, set operations, etc)