Plsql PL SQL+；使用替换变量的SQL简单语句

我想创建一个简单的IF-THEN-ELSE PL SQL，而不创建如下用于SQL工具的注释性块：

variable a varchar2(20) 
variable b varchar2(20) 

EXEC :a := '&insert_value_a'
EXEC :b := '&insert_value_b'

--下一部分我不知道怎么做：

如果给定了value:a并且它不是NULL，那么

EXECUTE
SELECT column1,column2,column3 
FROM table
WHERE condition
AND column2 = :a or column3 = :a;

ELSE

--如果给定值b，则使用该值获取值：a

EXECUTE
SELECT column2 INTO :a 
FROM table
WHERE column_x = :b;

SELECT column1,column2,column3 
    FROM table
    WHERE condition
    AND column2 = :a or column3 = :a;
END;

---

尝试使用NVL，如下所示：

SELECT column1,column2,column3 
  FROM table
 WHERE condition
   AND column2 = NVL(:a,SELECT column2
                          FROM table
                         WHERE column_x = :b)
    or column3 = NVL(:a,SELECT column2
                          FROM table
                        WHERE column_x = :b);