pl/sqlcoalesce/nvl

-- Select the first one which is not null:
example：
SQL> select coalesce(null,null,2) from dual;
COALESCE(NULL,NULL,2)
---------------------
                    2 

-- Two parameters are only required here:
select nvl(1, null) from dual;

-- If equivalent then null else return the first param 
select nullif(2,2) from dual; 


The following three sections are equivalent:


-- One, notice the difference between CASE statement and CASE expression
SET SERVEROUTPUT ON
DECLARE
    v_num1 NUMBER := &sv_num1;
    v_num2 NUMBER := &sv_num2;
    v_num3 NUMBER := &sv_num3;
    v_result NUMBER;
BEGIN
    v_result := CASE
            WHEN v_num1 IS NOT NULL THEN v_num1
            ELSE    
                CASE
                    WHEN v_num2 IS NOT NULL THEN v_num2
                    ELSE v_num3
                END
            END;
    DBMS_OUTPUT.PUT_LINE ('Result: '||v_result);
END; 
 
-- Two
CASE
    WHEN v_num1 IS NOT NULL THEN v_num1
    WHEN v_num2 IS NOT NULL THEN v_num2
    ELSE v_num3
END

-- Three
CASE
    WHEN v_num1 IS NOT NULL THEN v_num1
    ELSE COALESCE(v_num2, v_num3)
END