How can i display Mysql entries in a specific order - php

Question

In my android app i am trying to view a list of friends in the order of who is online and who is not
The code i am using to do it is extracting the request flag (1) from friends table and then use the friend_id to get data from users table and use json_encode to send data back to app in order to display it , the order of friends is displayed in the same order they are in users table :
Here is the code :
if(isset($_POST['list_friends'])) {
$sql = "SELECT friend_id FROM friends WHERE user_id='$user_id' && request='1'";
$res = mysqli_query($connection,$sql) or die(mysqli_error());
$row_to_json = array();
while($rows = mysqli_fetch_assoc($res)) {
foreach($rows AS $row) {
$sql2 = "SELECT * FROM users WHERE id='$row'
ORDER BY FIELD(is_online,'1') DESC";
$res2 = mysqli_query($connection,$sql2) or die(mysqli_error());
while ($rows2 = mysqli_fetch_assoc($res2)) {
//echo $rows_to_json = json_encode((object)$rows2);
$row_to_json[] = (($rows2));
}
}
}
echo json_encode(($row_to_json));
}
is_online field is either (0 if offline and 1 if online)
Any idea of what is wrong in this code ?
EDIT 1 :
I tried this code :
SELECT
friends.friend_id,users.is_online
FROM friends INNER JOIN users ON friends.user_id = users.id
WHERE
(friends.user_id = 21 and request = 1)
ORDER BY
(users.is_online) DESC
I added users.is_online to check for the actual output of this field and this is what is got :
Note that the actual is_online value exists in users table for **22** and
**27** is **0** not **1** !
EDIT 2 :
**
Perfect :)
**

Answer 1

I agree with #Fahad Anjum's comment in that && is very unusual as AND operator in a WHERE condition. Having said this:
1) You could leave away the FIELD() function completely, i.e. write is_online instead of FIELD(is_online, '1').
2) You are using strings where numbers would be sufficient (e.g. user_id = '$user_id' instead of user_id = $user_id). This might be worrying and might cost performance due to implicit casts MySQL might have to do in those cases, and when it comes to the FIELD() function, might be the problem.
The MySQL documentation states:
If all arguments to FIELD() are strings, all arguments are compared as
strings. If all arguments are numbers, they are compared as numbers.
Otherwise, the arguments are compared as double.
In your FIELD() function, you have a mixture of string ('1') and number (is_online) arguments (you have stated that 0 and 1 are possible values of is_online, i.e. is_online contains a number).
I am asking myself what happens when MySQL compares the string '1' with the number 1 after having converted both of them to double, or if something bad happens during the conversion. Theoretically, it should not be a problem because converting a number to double should give the same result as converting that stringified number to double, and because 1 can be represented exactly as a double, but I could imagine that we have a problem here. Unfortunately, I can't test it right now, but you could try writing FIELD(is_online, 1) instead of FIELD(is_online, '1').
3) The main problem is: In your first query, you select friends of the user with the given $user_id. Then you are looping through these friends and for every friend are executing a query against the user table to find out if that user (friend) is online.
That second query returns exactly one row each time; therefore, ordering is useless here.
You will have to solve this problem by doing a join between the friends and the users table in the first place.
Since I have never used PHP, I am a bit reluctant to suggest code here (I don't want to make a fool of myself by publishing code in a language I know nothing about without even being able to test). But your query would resemble the following:
SELECT
friends.friend_id
FROM
(friends INNER JOIN users ON friends.friend_id = users.id)
WHERE
(friends.user_id = $user_id)
ORDER BY
(users.is_online DESC)
Please note that I have left away your second condition here (request = '1') because I can't imagine what it means and it is not that relevant here. You can easily re-add it to the code above.