Question

I have two tables:

conversation_participants

id      | conversation_id | user_id
int(11) |  int(11)        |   int(11)

users

id      | username   | ....
int(11) | varchar(20)| ....

I am trying to select all the users which have a certain string in their username which are not (yet) participants in a particular conversation. Currently I have two SQL queries:

SELECT user_id FROM conversation_participants WHERE conversation_id=?

and

SELECT id, username from users WHERE username LIKE '%%%{$_GET['q']}%%' ORDER BY id DESC LIMIT 10

And remove all the results from the second query which have the same user_id as the ones from the first query. Is there any way to merge these two queries into one ?

Thank you in advance

Answer 1

SELECT id, username from users 
WHERE username LIKE '%%%{$_GET['q']}%%' 
and username not in 
(
  SELECT user_id FROM conversation_participants 
  WHERE conversation_id=users.conversation_id
)
ORDER BY id DESC LIMIT 10

Don’t build your sql statements concatenating strings. You should use some sort of utility method to sanitize your input. Your statement is prone to sql injection attacks.

PHP has several functions for this. Look into mysql_real_escape_string