Re-write table update as set instead of iterative method - oracle

Question

I have a list of user email addresses, in a test Oracle database, which are currently all set to the same value. I want to replace these with unique entries, with a number of invalid addresses and null values mixed in. My table currently looks like this and is around 250k rows in total.(I've excluded the null and invalid entries to save some space)
+-------------+--------------------+
| employee_id | email |
+-------------+--------------------+
| 1 | test#testemail.com |
| 2 | test#testemail.com |
| 3 | test#testemail.com |
|... |... |
+-------------+--------------------+
And I'd like it to look like this
+-------------+---------------------+
| employee_id | email |
+-------------+---------------------+
| 1 | test1#testemail.com |
| 2 | test2#testemail.com |
| 3 | test3#testemail.com |
|... |... |
+-------------+---------------------+
I've wrote the following PL/SQL to do the change, it works, but it's seems very inefficient. Can I use another method to take advantage of set processing?
Thanks for any help with this.
DECLARE
i number(20);
l_employee_id hr.employees.employee_id%TYPE;
output_query varchar2(1000);
CURSOR c_cursor IS
SELECT employee_id
FROM hr.employees;
PROCEDURE update_sql(id_num IN hr.employees.employee_id%TYPE,
email IN VARCHAR2) IS
BEGIN
output_query := 'UPDATE hr.employees
SET email = '''|| email ||'''
WHERE employee_id = '|| id_num;
dbms_output.put_line(output_query); --for debug
EXECUTE IMMEDIATE output_query;
END;
BEGIN
OPEN c_cursor;
i := 1;
<<outer_loop>>
LOOP
For j IN 1..5 LOOP
FETCH c_cursor INTO l_employee_id;
EXIT outer_loop WHEN c_cursor%NOTFOUND;
IF j <= 3 THEN
update_sql(l_employee_id, ('test' || i || '#testemail.com'));
ELSIF j = 4 THEN
update_sql(l_employee_id, ('test' || i || 'testemail.com'));
ELSIF j = 5 THEN
update_sql(l_employee_id, ' ');
END IF;
i := i + 1;
END LOOP;
END LOOP outer_loop;
CLOSE c_cursor;
END;
/
EDIT - 26/09/2016 - to clarify size of table.

Answer 1

create table emp (emp_id number, email varchar2(32));
insert into emp select level as emp_id, 'test#testemail.com' as email
from dual connect by level<=2500000;
update emp set email = regexp_replace(email, '(\w+)(#\w+\.\w+)', '\1' || emp_id || '\2');
--250,000 rows updated ~16 sec.
EMP_ID, EMAIL
1 test1#testemail.com
2 test2#testemail.com
3 test3#testemail.com
...
drop table emp;

Answer 2

First off, even if you were going to code this iteratively, please don't use dynamic SQL where it is not necessary. And it is only necessary if you don't know the table or columns you are going to be querying at compile time.
That said, it sounds like you just want
UPDATE employees
SET email = (case when employee_id <= 3
then 'test' || employee_id || '#testemail.com'
when employee_id = 4
then 'test' || employee_id || 'testemail.com'
when employee_id = 5
then ' '
else null
end)

Answer 3

Oracle Setup:
create table employees (
id NUMBER,
email VARCHAR2(100)
);
INSERT INTO employees
SELECT 1, 'test#testemail.com' FROM DUAL UNION ALL
SELECT 2, 'test#testemail.com' FROM DUAL UNION ALL
SELECT 3, 'test#testemail.com' FROM DUAL UNION ALL
SELECT 4, 'test#testemail.com' FROM DUAL UNION ALL
SELECT 5, 'test#testemail.com' FROM DUAL UNION ALL
SELECT 6, 'test#testemail.com' FROM DUAL;
Query:
update employees
set email = CASE MOD( ROWNUM, 5 )
WHEN 4 THEN 'test' || ROWNUM || 'testemail.com'
WHEN 0 THEN ''
ELSE 'test' || ROWNUM || '#testemail.com'
END;
Output:
SELECT * FROM employees;
ID EMAIL
---------- ------------------------
1 test1#testemail.com
2 test2#testemail.com
3 test3#testemail.com
4 test4testemail.com
5
6 test6#testemail.com