Why are arrays indexed by datatype?

TYPE  emp_type_rec IS TABLE OF
employees%rowtype
INDEX BY BINARY_INTEGER;

When we define an array, why is it indexed b开发者_高级运维y some datatype?

Because you have a choice of data types to index by. For example:

DECLARE
    TYPE empno_rec_type IS TABLE OF employees.empno%type INDEX BY VARCHAR2(10);
    empno_rec empno_rec_type;
BEGIN
    empno_rec('SMITH') = 123;
    empno_rec('JONES') = 456;
END;