Why is this check for null associative array in PL/SQL failing? - oracle

Question

I have an associative array created by a type of rowtype of a table column.
To give an example, this is how it is(the table names are different, but the structure is the same):
This is the DDL of the table
CREATE TABLE employees
(
id NUMBER,
name VARCHAR2(240),
salary NUMBER
);
Here's what my procedure is doing:
DECLARE
TYPE table_of_emp
IS TABLE OF employees%ROWTYPE INDEX BY BINARY_INTEGER;
emp TABLE_OF_EMP;
BEGIN
IF emp IS NULL THEN
dbms_output.Put_line('Null associative array');
ELSE
dbms_output.Put_line('Not null');
END IF;
END;
I assume this should result in "Null associative array" being printed. However, the if condition fails and the execution jumps to the else part.
Now if I put in a for loop to print the collection values
DECLARE
TYPE table_of_emp
IS TABLE OF employees%ROWTYPE INDEX BY BINARY_INTEGER;
emp TABLE_OF_EMP;
BEGIN
IF emp IS NULL THEN
dbms_output.Put_line('Null associative array');
ELSE
dbms_output.Put_line('Not null');
FOR i IN emp.first..emp.last LOOP
dbms_output.Put_line('Emp name: '
|| Emp(i).name);
END LOOP;
END IF;
END;
then the program unit raises an exception, referencing the for loop line
ORA-06502: PL/SQL: Numeric or value error
which I presume is because of the null associative array. Is the error being raised because of null associative array?
So why is the first check failing then? What am I doing wrong?
The database server is Oracle 11g EE (version 11.2.0.3.0 64 bit)

Answer 1

I assume this should result in "Null associative array" being printed. That assumption is wrong for associative arrays. They exist when declared, but are empty. It would be correct for other types of PL/SQL collections:
Until you initialize it, a nested table or varray is atomically null;
the collection itself is null, not its elements. To initialize a
nested table or varray, you use a constructor, a system-defined
function with the same name as the collection type. This function
constructs collections from the elements passed to it.
You must explicitly call a constructor for each varray and nested
table variable. Associative arrays, the third kind of collection, do
not use constructors. Constructor calls are allowed wherever function
calls are allowed. Initializing and Referencing Collections
Compare:
SQL> declare
2 type varchar2_100_aa is table of varchar2(100) index by binary_integer;
3 test varchar2_100_aa;
4 begin
5 test(1) := 'Hello';
6 dbms_output.put_line(test(1));
7 end;
8 /
Hello
PL/SQL procedure successfully completed.
SQL> declare
2 type varchar2_100_va is varray(100) of varchar2(100);
3 test varchar2_100_va;
4 begin
5 test(1) := 'Hello';
6 dbms_output.put_line(test(1));
7 end;
8 /
declare
*
ERROR at line 1:
ORA-06531: Reference to uninitialized collection
ORA-06512: at line 5
Variable array done correctly:
SQL> declare
2 type varchar2_100_va is varray(10) of varchar2(100);
3 test varchar2_100_va;
4 begin
5 test := varchar2_100_va(); -- not needed on associative array
6 test.extend; -- not needed on associative array
7 test(1) := 'Hello';
8 dbms_output.put_line(test(1));
9 end;
10 /
Hello
PL/SQL procedure successfully completed.
Because the associative array is empty first and last are null, which is why your second example results in ORA-06502: PL/SQL: Numeric or value error:
SQL> declare
2 type varchar2_100_aa is table of varchar2(100) index by binary_integer;
3 test varchar2_100_aa;
4 begin
5 dbms_output.put_line(test.count);
6 dbms_output.put_line(coalesce(to_char(test.first), 'NULL'));
7 dbms_output.put_line(coalesce(to_char(test.last), 'NULL'));
8 test(1) := 'Hello';
9 dbms_output.new_line;
10 dbms_output.put_line(test.count);
11 dbms_output.put_line(coalesce(to_char(test.first), 'NULL'));
12 dbms_output.put_line(coalesce(to_char(test.last), 'NULL'));
13 end;
14 /
0
NULL
NULL
1
1
1
PL/SQL procedure successfully completed.
EDIT Also note that associative arrays can be sparse. Looping over the numbers between first and last will raise an exception for any collection that is sparse. Instead use first and next like so: (Last and prev to loop the other direction.)
SQL> declare
2 type varchar2_100_aa is table of varchar2(100) index by binary_integer;
3 test varchar2_100_aa;
4 i binary_integer;
5 begin
6 test(1) := 'Hello';
7 test(100) := 'Good bye';
8 dbms_output.put_line(test.count);
9 dbms_output.put_line(coalesce(to_char(test.first), 'NULL'));
10 dbms_output.put_line(coalesce(to_char(test.last), 'NULL'));
11 dbms_output.new_line;
12 --
13 i := test.first;
14 while (i is not null) loop
15 dbms_output.put_line(to_char(i, '999') || ' - ' || test(i));
16 i := test.next(i);
17 end loop;
18 end;
19 /
2
1
100
1 - Hello
100 - Good bye
PL/SQL procedure successfully completed.

Answer 2

I'm not going to answer why the first check is failing. I've never thought of doing anything like that and am quite surprised that it doesn't raise an error.
The reason why you're getting an exception raised on the loop is, as you've noted, that the index emp.first does not exist.
Rather than checking for nulls, you should really be checking for the existence of this index. Which you can do be using the .exists(i) syntax:
if not emp.exists(emp.first) then
dbms_output.put_line('Nothing in here.');
end if;