COLLECTIONS in Oracle

Index-By Tables (Associative Arrays) -  Same as arrays except that have no upper bounds, allowing them to constantly extend. As the name implies, the collection is indexed using BINARY_INTEGER values, which do not need to be consecutive. The collection is extended by assigning values to an element using an index value that does not currently exist.

TYPE table_type IS TABLE OF NUMBER(10)
    INDEX BY BINARY_INTEGER;

Nested Table Collections - Nested table collections are an extension of the index-by tables. The main difference between the two is that nested tables can be stored in a database column but index-by tables cannot. In addition some DML operations are possible on nested tables when they are stored in the database. During creation the collection must be dense, having consecutive subscripts for the elements. Once created elements can be deleted using the DELETE method to make the collection sparse. The NEXT method overcomes the problems of traversing sparse collections.

TYPE table_type IS TABLE OF NUMBER(10);

Varray Collections - A VARRAY is similar to a nested table except you must specifiy an upper bound in the declaration. Like nested tables they can be stored in the database, but unlike nested tables individual elements cannot be deleted so they remain dense.

TYPE table_type IS VARRAY(5) OF NUMBER(10);

Assignments and Equality Tests

Same type of collections - 
  TYPE table_type IS TABLE OF NUMBER(10);
  v_tab_1  table_type;
  v_tab_2  table_type;

  v_tab_2 := v_tab_1; -- This works

  IF v_tab_1 = v_tab_2 THEN -- This works
    ....
  END IF;


Different type of collections - 
  
  TYPE table_type_1 IS TABLE OF NUMBER(10);
  TYPE table_type_2 IS TABLE OF NUMBER(10);
  v_tab_1  table_type_1;
  v_tab_2  table_type_2;

  v_tab_2 := v_tab_1;  -- This will throw error

  IF v_tab_1 = v_tab_2 THEN  -- This will throw error
    ....
  END IF;


Collection Methods:

A variety of methods exist for collections, but not all are relevant for every collection type.

EXISTS(n) - Returns TRUE if the specified element exists.
COUNT - Returns the number of elements in the collection.
LIMIT - Returns the maximum number of elements for a VARRAY, or NULL for nested tables.
FIRST - Returns the index of the first element in the collection.
LAST - Returns the index of the last element in the collection.
PRIOR(n) - Returns the index of the element prior to the specified element.
NEXT(n) - Returns the index of the next element after the specified element.
EXTEND - Appends a single null element to the collection.
EXTEND(n) - Appends n null elements to the collection.
EXTEND(n1,n2) - Appends n1 copies of the n2th element to the collection.
TRIM - Removes a single element from the end of the collection.
TRIM(n) - Removes n elements from the end of the collection.
DELETE - Removes all elements from the collection.
DELETE(n) - Removes element n from the collection.
DELETE(n1,n2) - Removes all elements from n1 to n2 from the collection.

MULTISET Operations

  TYPE t_tab IS TABLE OF NUMBER;
  l_tab1 t_tab := t_tab(1,2,3,4,5,6);
  l_tab2 t_tab := t_tab(5,6,7,8,9,10);
  ...

MULTISET UNION {ALL | DISTINCT} Operator - 

  l_tab1 := l_tab1 MULTISET UNION l_tab2;

  l_tab1 := l_tab1 MULTISET UNION DISTINCT l_tab2;

MULTISET EXCEPT {DISTINCT} Operator

  l_tab1 := l_tab1 MULTISET EXCEPT l_tab2;

MULTISET INTERSECT {DISTINCT} Operator

  l_tab1 := l_tab1 MULTISET INTERSECT l_tab2;

https://oracle-base.com/articles/8i/collections-8i
https://docs.oracle.com/cloud/latest/db112/LNPLS/composites.htm#LNPLS005