Explain the three PL/SQL collection types and when to use each

Associative array (TABLE OF ... INDEX BY PLS_INTEGER or VARCHAR2): no initialisation or EXTEND, can be sparse, cannot be a table column or used directly in SQL — ideal as an in-memory lookup or a FORALL/BULK COLLECT buffer. Nested table (TABLE OF ...): starts empty/atomically null, needs a constructor and EXTEND, can be dense or sparse, can be a column and used with MULTISET/TABLE(). VARRAY (VARRAY(n) OF ...): bounded maximum size, always dense, order preserved, good for a short fixed list stored inline in a row.

DECLARE
  TYPE map_t   IS TABLE OF NUMBER INDEX BY VARCHAR2(30);  -- associative
  TYPE ntab_t  IS TABLE OF NUMBER;                        -- nested table
  TYPE varr_t  IS VARRAY(5) OF NUMBER;                    -- varray
  m  map_t;
  n  ntab_t := ntab_t();
  v  varr_t := varr_t(10, 20, 30);
BEGIN
  m('EUR') := 1.09;
  n.EXTEND; n(1) := 42;
  DBMS_OUTPUT.PUT_LINE(m('EUR') || ' ' || n(1) || ' ' || v(2));
END;
/