UNION query functionality and the query structure - mysql

Question

What I'm trying to achieve is to insert values into a table if that value does not exist in 2 extra tables.
INSERT INTO visitor(
visitor_username,
email,
PASSWORD
)
SELECT * FROM
(
SELECT
'admin2000',
'adminemail#mail.com',
'123456'
) AS tmp
WHERE NOT EXISTS
(
SELECT
admin.admin_username,
admin.email
FROM
admin AS admin
WHERE
admin.admin_username = 'admin2000' AND admin.email =
'adminemail#mail.com'
UNION
SELECT
staff.staff_username,
staff.email
FROM
staff AS staff
WHERE
staff.staff_username = 'admin2000' AND staff.email =
'adminemail#mail.com'
)
LIMIT 1
In the WHERE NOT EXIST part when I only ask for *_username (example: admin_username or staff_username) it works well, but when I need to verify if the email exists too it does not work as intended.
Am I using WHERE NOT EXIST properly? if the username 'admin2000' and email 'adminemail#mail.com' exist on the table 'admin' and I'm trying to insert it into 'visitor' table it inserts it and it should not be doing that.

Answer 1

The problem is the AND in the subqueries. It searches for rows that have that username AND that email. So if you have an admin called admin2000, but with a different e-mail address, that admin won't be returned by the subquery, and so the new row will be inserted.
Use OR instead of AND, and the the problem will be solved.

Answer 2

You seem to want to be writing a query like this:
INSERT INTO visitor (visitor_username, email, PASSWORD)
SELECT t.*
FROM (SELECT 'admin2000' as visitor_username, 'adminemail#mail.com' as email, '123456' as PASSWORD
) t
WHERE NOT EXISTS (SELECT 1
FROM admin a
WHERE a.visitor_username = t.visitor_username AND a.email = t.email
)
UNION
SELECT s.staff_username, s.email, ? as password
FROM staff s
WHERE s.staff_username = 'admin2000' AND s.email =
'adminemail#mail.com';
Note that the second subquery is missing a password, so there is an error.
This seems more concisely written using a single query:
INSERT INTO visitor (visitor_username, email, PASSWORD)
SELECT t.*
FROM (SELECT 'admin2000' as visitor_username, 'adminemail#mail.com' as email, '123456' as PASSWORD
) t
WHERE NOT EXISTS (SELECT 1
FROM admin a
WHERE a.admin_username = t.visitor_username AND a.email = t.email
) AND
NOT EXISTS (SELECT 1
FROM staff s
WHERE s.staff_username = t.visitor_username AND s.email = t.email
);