Assigning and Comparing Collections

One collection can be assigned to other by an SELECT, INSERT, UPDATE, or FETCH statement, an assignment statement, or by a subprogram call. As the illustration shown below, the collections should have the same datatype. Having the similar element type is not enough.

DECLARE
TYPE Clientele IS VARRAY(100) OF Customer;
TYPE Vips IS VARRAY(100) OF Customer;
group1 Clientele := Clientele(...);
group2 Clientele := Clientele(...);
group3 Vips := Vips(...);
BEGIN
group2 := group1;
group3 := group2; -- illegal;


Various datatypes becomes automatically null (and should be reinitialized). Now consider the illustration as shown below:

 
DECLARE
TYPE Clientele IS TABLE OF Customer;
group1 Clientele := Clientele(...); -- initialized
group2 Clientele; -- atomically null
BEGIN
IF group1 IS NULL THEN ... -- condition yields FALSE
group1 := group2;
IF group1 IS NULL THEN ... -- condition yields TRUE
...
END;


Similarly, if you assign the non-value NULL to a collection, the collection becomes automatically null.


Assigning Collection Elements

You can assign the value of an expression to the specific element in a collection by using the syntax

collection_name(subscript) := expression;



Where the expression yields a value of the type specified for elements in the collection type definition. If the subscript is null or not convertible to an integer, the PL/SQL raises the predefined exception VALUE_ERROR. If the collection is automatically null, then the PL/SQL raises COLLECTION_IS_NULL. Some of the examples are shown below:




DECLARE
TYPE NumList IS TABLE OF INTEGER;
nums NumList := NumList(10,20,30);
ints NumList;
...
BEGIN
...
nums(1) := TRUNC(high/low);
nums(3) := nums(1);
nums(2) := ASCII(’B’);
/* Assume execution continues despite the raised exception. */
nums(’A’) := 40; -- raises VALUE_ERROR
ints(1) := 15; -- raises COLLECTION_IS_NULL
END;


Comparing Whole Collections

The Nested tables and varrays can be automatically null; therefore they can be tested for the nullity, as the example below shows:

DECLARE
TYPE Staff IS TABLE OF Employee;
members Staff;
BEGIN
...
IF members IS NULL THEN ... -- condition yields TRUE;
END;


Though, the collections cannot be compared for equality or inequality. For illustration, the IF condition below is illegal as shown:

DECLARE
TYPE Clientele IS TABLE OF Customer;
group1 Clientele := Clientele(...);
group2 Clientele := Clientele(...);
BEGIN
...
IF group1 = group2 THEN -- causes compilation error
...
END IF;
END;
This restriction also applies to implicit the comparisons. For illustration, the collections cannot appear in an ORDER BY, GROUP BY, or DISTINCT list.