Single SQL query for selecting chats - mysql

Question

I need to select all chats and right now I'm selecting them using multiple queries like this:
$query_1 = mysqli_query($database, "SELECT chat_id, user_id_1, user_id_2
FROM chat
WHERE user_id_1 = '$user_id' OR user_id_2 = '$user_id')");
if (mysqli_num_rows($query_1) > 0) {
while ($row = mysqli_fetch_assoc($query_1)) {
$chat_id = $row['chat_id'];
$user_id_1 = $row['user_id_1'];
$user_id_2 = $row['user_id_2'];
if ($user_id_1 == $user_id)
$user_id_chat = $user_id_2;
else
$user_id_chat = $user_id_1;
$query_2 = mysqli_query($database, "SELECT username
FROM user
WHERE user_id = '$user_id_chat'");
$row2 = mysqli_fetch_assoc($query_2);
$username = $row2['username'];
$query_3 = mysqli_query($database, "SELECT text,
(SELECT COUNT(message_id) FROM message WHERE chat_id = '$chat_id' AND user_id != '$user_id' AND seen = '0') AS unread_messages
FROM message
WHERE chat_id = '$chat_id'
ORDER BY message_id DESC
LIMIT 1");
$row3 = mysqli_fetch_assoc($query_3);
$text = $row3['text'];
$unread_messages = $row3['unread_messages'];
}
}
Is there a way to get all this ($chat_id, $user_id_chat, $username, $text and $unread_messages) using only one query?
EDIT: Added example data and expected output:
Table user
user_id username ...
1 User1 ...
2 User2 ...
3 User3 ...
4 User4 ...
Table chat
chat_id user_id_1 user_id_2 ...
1 3 4 ...
2 1 3 ...
Table message
message_id chat_id user_id text seen ...
1 1 4 Hi! 1 ...
2 2 1 Hello 1 ...
3 1 3 hi 0 ...
4 2 3 Hi 0 ...
5 2 3 How are you? 0 ...
If $user_id is 1 there should be only one chat and value of $chat_id should be 2, of $user_id_chat should be 3, of $username should be 'User3', of $text should be 'How are you?' and of $unread_messages should be 2.

Answer 1

http://sqlfiddle.com/#!9/8c6cb/8
$query_1 = mysqli_query($database,
"SELECT
c.chat_id,
u.username,
COALESCE(m.text,'MY PLACEHOLDER'),
COALESCE(um.unread_messages,0)
FROM (
SELECT chat_id,
IF(user_id_1 = '$user_id', user_id_2, user_id_1) user_id_chat
FROM chat
WHERE user_id_1 = '$user_id' OR user_id_2 = '$user_id'
) c
LEFT JOIN user u
ON u.user_id = c.user_id_chat
LEFT JOIN (SELECT chat_id, SUM(seen = '0',1,0) unread_messages, MAX(message_id) max_id
FROM message
WHERE user_id != '$user_id'
GROUP BY chat_id) um
ON um.chat_id = c.chat_id
LEFT JOIN message m
ON m.chat_id = c.chat_id
AND m.message_id = um.max_id ");
if (mysqli_num_rows($query_1) > 0) {
while ($row = mysqli_fetch_assoc($query_1)) {
$chat_id = $row['chat_id'];
$username = $row['username'];
$text = $row ['text'];
$unread_messages = $row['unread_messages'];
}
}

Answer 2

So, you are looking for a way to select from two tables in one query. Then you might like INNER JOIN.
Visit this link:
http://www.w3schools.com/sql/sql_join_inner.asp
OR
Make a function to select like this:
` function select ($query ="SELECT * FROM $table", $limit = 1) {
$row = mysql_query($query);
// I'm so lazy to type the other codes, just put ur while loop and other stuff(if any) here
} `
This then must be called as:
` select("SELECT chat_id, user_id_1, user_id_2
FROM chat
WHERE user_id_1 = '$user_id' OR user_id_2 = '$user_id')");
select(" (SELECT COUNT(message_id) FROM message WHERE chat_id = '$chat_id' AND user_id != '$user_id' AND seen = '0') AS unread_messages
FROM message
WHERE chat_id = '$chat_id' AND user_id != '$user_id'
ORDER BY message_id DESC
LIMIT 1"); `
This won't need multiple where loops.
Sorry for bad code layout, I'm a newbie here.
Good Luck!