Combining and/or statements crashing database - mysql

Question

I have the following mysql select. It gets the list of the users qualified on the event and it also gets some results from the users table.
SELECT event.*,
users.first_name,
users.last_name,
users.login,
users.email,
users.f_phone as userphone,
users.f_phone_code
FROM event,
users
WHERE (users.id=event.h_user and f_code LIKE %NEW%)
OR event.h_user = -1
ORDER BY event.`h_id` DESC
Everything worked perfectly, until I tried to get more results by adding the records with h_user = -1. There is not a lot of them, but I guess my query isn't good enough while the database of users is really big. The added OR statement is crashing database while getting results. I tried to add the brackets in WHERE statement but they arent helping.
Would you mind to help me on this please?

Answer 1

It happens because your implicit join conditions become messed up when u add or in the where clause, thus the query doesn't work as you expect it to. Try using explicit join instead (u can make the query work both ways, but explicit notation is just more readable) and leave all the filtering in the where clause:
from event
join users on users.id = event.h_user
where f_code like '%NEW%' or event.h_user = -1
Which would look like this using the implicit notation (don't use it, it is deprecated):
from event, users
where ( users.id = event.h_user ) and
( f_code like '%NEW%' or event.h_user = -1 )

Answer 2

You could also do this with joins and a UNION:
SELECT event.*,
users.first_name,
users.last_name,
users.login,
users.email,
users.f_phone as userphone,
users.f_phone_code
FROM event
INNER JOIN users ON users.id=event.h_user
WHERE f_code LIKE %NEW%
UNION
SELECT event.*,
users.first_name,
users.last_name,
users.login,
users.email,
users.f_phone as userphone,
users.f_phone_code
FROM event
INNER JOIN users ON users.id=event.h_user
WHERE event.h_user = -1
ORDER BY event.`h_id` DESC