Disclaimer: If you have important data on your server, backup before you perform any of these operations. I did not exhaustively test my solution.

To answer this, I did the following on Windows and Linux

(example shown is for Linux)

• Logged on - selected from user table.

• Deleted all users except the root one with password.

• Logged out, then logged back in again.

• Checked that I could perform various operations - they all worked (more than shown - e.g. create table).

So, the answer to your question is that yes, you can do what you ask.

But, I would also say that logging on as root on a regular basis is poor practice (if your data is important). If it's a dev/test server, that's not an issue.

Operations Performed.

mysql> select host, user, password from user;
+-----------------------+----------+-------------------------------------------+
| host                  | user     | password                                  |
+-----------------------+----------+-------------------------------------------+
| localhost             | root     | *381AD08BBFA647B14C82AC1094A29AD4D7E4F51D |
| localhost.localdomain | root     |                                           |
| 127.0.0.1             | root     |                                           |
| ::1                   | root     |                                           |
| localhost             |          |                                           |
| localhost.localdomain |          |                                           |
| localhost             | bugs     | *381AD08BBFA647B14C82AC1094A29AD4D7E4F51D |
| %                     | bugs     |                                           |
| localhost             | puser    | *9F8D1F4C5ECC93C1D04F6BF04508FEDE4A9CF7F3 |
| localhost             | MY_USER  | *B678380CFA4459BB48E7B66AE1341F5F5B97D14E |
| localhost             | testuser | *00E247AC5F9AF26AE0194B41E1E769DEE1429A29 |
| 12.23.*.*             |          |                                           |
| 85.*.*.*              |          |                                           |
+-----------------------+----------+-------------------------------------------+
13 rows in set (0.00 sec)

mysql> 

So then I did (deleting all records except the first one).

mysql> delete from user where host <> 'localhost';
Query OK, 7 rows affected (0.00 sec)
mysql> delete from user where user <> 'root';
Query OK, 5 rows affected (0.00 sec)

And the I selected from the user table.

mysql> select host, user, password from user;
+-----------+------+-------------------------------------------+
| host      | user | password                                  |
+-----------+------+-------------------------------------------+
| localhost | root | *381AD08BBFA647B14C82AC1094A29AD4D7E4F51D |
+-----------+------+-------------------------------------------+
1 row in set (0.00 sec)

Exited and then logged in again.

[pol@localhost inst]$ !904
./bin/mysql --defaults-file=./my.cnf -S ./mysql.sock -u root -p
Enter password:     
mysql> use test;

Database changed
mysql> show tables like 'z%';
+---------------------+
| Tables_in_test (z%) |
+---------------------+
| zsample             |
| zz                  |
+---------------------+
2 rows in set (0.02 sec)

mysql> 

So, I can still select - but can I perform other operations?

mysql> select * from zsample;
+------+
| x    |
+------+
|    3 |
|    4 |
+------+
2 rows in set (0.00 sec)

mysql> delete from zsample where x = 3;
Query OK, 1 row affected (0.05 sec)

mysql> select * from zsample;
+------+
| x    |
+------+
|    4 |
+------+
1 row in set (0.00 sec)