How does SQL UPDATE match more rows than similar SELECTs?

Question

Below, the SELECT query and the UPDATE query have the same FROM clause and WHERE clause. Thus, I expected the SELECT query to return the same number of rows that were matched and affected by the UPDATE query, but apparently this is not the case.

I don’t know if this is related, but, as seen from the update, I try to save the username and email address in another column, but, as seen from the last request, I do not save the message, just the username.

Why is this so?

mysql> SELECT p.old_username, p.username, p.old_email, u.email
    -> FROM users u INNER JOIN people p ON p.id = u.id INNER JOIN entities e ON e.id = p.id
    -> WHERE u.id =1753671666 AND u.id !=506836355 AND e.sites_id =2846702735;
+--------------+------------+-----------+---------------------+
| old_username | username   | old_email | email               |
+--------------+------------+-----------+---------------------+
| NULL         | myusername | NULL      | [e-mail] |
+--------------+------------+-----------+---------------------+
1 row in set (0.00 sec)

mysql>
mysql> UPDATE
    -> users u INNER JOIN people p ON p.id = u.id INNER JOIN entities e ON e.id = p.id
    -> SET p.old_username = p.username, p.username = NULL,
    -> p.old_email = u.email, u.email = NULL,
    -> e.record_status = 'inactive', e.date_modified = NOW( ), e.modified_by_id =506836355
    -> WHERE u.id =1753671666 AND u.id !=506836355 AND e.sites_id =2846702735;
Query OK, 3 rows affected (0.00 sec)
Rows matched: 3  Changed: 3  Warnings: 0

mysql>
mysql> SELECT p.old_username, p.username, p.old_email, u.email
    -> FROM users u INNER JOIN people p ON p.id = u.id INNER JOIN entities e ON e.id = p.id
    -> WHERE u.id =1753671666 AND u.id !=506836355 AND e.sites_id =2846702735;
+--------------+----------+-----------+-------+
| old_username | username | old_email | email |
+--------------+----------+-----------+-------+
| myusername   | NULL     | NULL      | NULL  |
+--------------+----------+-----------+-------+
1 row in set (0.00 sec)

mysql>

Answer 1

The SELECT statement reports that one row is the result of a combined query.

3:

p.old_username (and several others)
e.date_modified (and others)
u.email

3 .