PHP查询多个表

Question

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'];


 }

Answer 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 *.

Answer 2

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'];


 }

Answer 3

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'];

Answer 4

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'];

}

Answer 5

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

<?php
  $sql = "SELECT * FROM comments as c INNER JOIN users as u ON  c.user_id=u.user_id WHERE u.user_id=:user_id ORDER BY c.date DESC LIMIT 10";
  

  //prepare the connection to database
  $prep = $conn->prepare($sql);

  //change the parameters for user_id on WHERE condition you can put anything you want but i will put the user session id
  $prep->bindParam(":user_id", $_SESSION['user_id']);
  $prep->execute();

 //ill use fetchAll function because it can be more than 1 comment
 $datas = $prep->fetchAll();

foreach($datas as $data){
  echo $data['user_id'];
  echo $data['comment'];
  echo $data['date'];
}

?>