Index-by tables

Student table

STUDENT_ID FIRST_NAME LAST_NAME CITY
1 Daniel SCOTT New York
2 Anthony SIMMONS Chicago
3 Sophia THOMPSON Los Angeles
4 Emily PETERSON Phoenix
5 David DAWSON Seattle
6 Gabriel LEWIS Boston
7 Natalie MARTIN Baltimore
8 Ava GONZALES Pasadena
9 Elliot PETERSON Chicago

Index-by tables(associative arrays) 

DECLARE
	TYPE type_student IS TABLE OF VARCHAR2(200) INDEX BY VARCHAR2(200);
	adr_list type_student;
	v_address   VARCHAR2(200);
BEGIN
	adr_list('Los Angeles') := 'address_1';
	adr_list('Chicago') := 'address_2';
	adr_list('Seattle') := 'address_3';
	v_address := adr_list.FIRST;
	WHILE v_address IS NOT null LOOP 
		FOR x in (SELECT * FROM students WHERE city=v_address)
		LOOP
			DBMS_OUTPUT.PUT_LINE (x.student_id||' - '||x.first_name||' '||x.last_name||' - '||x.city);      
		END LOOP;
		v_address := adr_list.NEXT(v_address);
	END LOOP;
END;
/

Output

2 – Anthony SIMMONS – Chicago
9 – Elliot PETERSON – Chicago
3 – Sophia THOMPSON – Los Angeles
5 – David DAWSON – Seattle