Basic Oracle Concepts and Programming Question:

Answer #1
When a variable name collides with a column name, PL/SQL will use it as the variable if it is used where variable is allowed; It will be used as the column, if it is used where variable is not allowed but column is allowed. Here is a good example of variable names collide with column names:

CREATE TABLE student (id NUMBER(5) PRIMARY KEY,
first_name VARCHAR(80) NOT NULL,
last_name VARCHAR(80) NOT NULL);
Table created.

DECLARE
id NUMBER;
first_name CHAR(10);
BEGIN
id := 29;
first_name := 'Bob';
INSERT INTO student VALUES(id, first_name, 'Henry');
first_name := 'Joe';
INSERT INTO student VALUES(id+1, first_name, 'Bush');

first_name := 'ggl';
UPDATE student SET first_name = first_name WHERE id = 29;
-- 1 row updated
-- Both 'first_name's are treated as column names

UPDATE student SET first_name = first_name
WHERE id = id+1;
-- 0 rows updated
-- Both "id"s are treated as variable names

DELETE FROM student WHERE id = id;
-- 2 rows deleted
END;
/

Answer #2
SELECT * FROM student;
0 rows selected

Noticed that "id = id+1" in the WHERE clause will be evaluated to FALSE, because both "id"s are treated as variables. Similarly "id = id" will also be evaluated to TRUE in the WHERE clause. But both "first_name"s in the SET clause will be treated as column names.