Hướng dẫn php multiple table query

I'm having trouble getting any information to display from this query. Anyone know where I'm going wrong?

Thank you!

 $query =   "SELECT * ".
            "FROM comments, users ".
            "WHERE comments.user_id = users.user_id ".
            "ORDER BY comments.date DESC ".
            "LIMIT 10";

$result = mysql_query[$query] or die[mysql_error[]];

   while [$row = mysql_fetch_array[$result]] {


  echo $row['users.user_id'];
  echo $row['comments.comment'];


 }

OMG Ponies

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asked Oct 26, 2009 at 20:35

1

use mysql_fetch_assoc[] instead of mysql_fetch_array[]. In your loop use the column name as the array key:

while [$row = mysql_fetch_assoc[$result]] {

  echo $row['column_name1'];
  echo $row['column_name1'];

}

In your query try to be more specific on the select statement, try not to use *.

answered Oct 26, 2009 at 20:44

Hubert PerronHubert Perron

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You're probably getting the error because you are sorting [ORDER BY] on a field that does not exist in your query.

It would be best practice to not use the "SELECT *" querying. If all you need are specific values, specify them. This also helps when retrieving the data...

$query =   "SELECT users.user_id, comments.comment, comments.date ".
                        "FROM comments, users ".
                        "WHERE comments.user_id = users.user_id ".
                        "ORDER BY comments.date DESC ".
                        "LIMIT 10";

$result = mysql_query[$query] or die[mysql_error[]];

   while [$row = mysql_fetch_array[$result]] {


  echo $row['user_id'];
  echo $row['comment'];
  echo $row['date'];


 }

answered Oct 26, 2009 at 20:50

It is good practice to specify column names in a query rather than using * - on some DBs there is a performance impact and on all it prevents any unexpected behaviour cropping up from table changes.

In the example I think the issue is arsing from the array keys you are using - you don't need to include the table name in them, just the column name:

  echo $row['user_id']; 
  echo $row['comment'];

answered Oct 26, 2009 at 20:51

MacrosMacros

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The better practice is to write only fields what you need in your sql query like this:

$query =   "SELECT u.user_id uid, c.comment comment ".
                    "FROM comments c, users u ".
                    "WHERE comments.user_id = users.user_id ".
                    "ORDER BY comments.date DESC ".
                    "LIMIT 10";

Using so type of queries you reduce the time of executing your query and transmitting data from database server to your php script. After this modification your cycle transformed to:

while [$row = mysql_fetch_array[$result]] {
 echo $row['uid'], $row['comment'];

}

answered Oct 26, 2009 at 20:54

I use PDO method but this can work for you too:

answered Feb 7, 2021 at 16:52

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