如何检查表中是否存在元素（mysql）

Question

I have create a database Mysql and I am using in php. I want for each user to be checked if there is data.

I have three tables. In the table User has a unique value called ID_u and isn't auto_increment, the table Application has a unique value called ID(auto_increment) and table InformationUser has a unique value called ID(auto_increment). The tables Users and InformationUser joins with table Application.

ID_u = a user with id_u-> 1234

I'am try this:

if( ID_u  exists data){
    location1.php ->data
}else{
    location2.php -> input information
}

I would be thankful for any help!!

Answer 1

I'm not sure I understand the structure of your database, but you may take a look at this:

$query = "
SELECT * FROM users u
LEFT JOIN application a
ON u.ID_u = a.ID_u
LEFT JOIN informationuser i
ON a.ID = i.ID
//Here you can specify 'WHERE u.ID_u IN (1,2,3,4)'
GROUP BY i.ID
" ;


$users = array() ; //Create storage for users. Use their ID as indeces
$result = $mysqli->query($query) ;
if ($result && $result->num_rows >= 1){
  while ($row = $result->fetch_assoc()){
    if (!isset($users[$row['ID_u'])){
      $users[$row['ID_u']] = array() ; //Create array for a new user
    }
    $users[$row['ID_u']][] = array(  //Fill array in with data and add it to user.
      "email" => $row["email"],
      "phone" => $row["phone"]
    ) ;
  }
}

Afterwards, let's check if data exists for a user with ID = 3 (if 3 is an int ):

if (!empty($users[3])){
  echo "Yeah, data exists for user with ID 3" ;
}

Answer 2

First of all, if the user can only have one set of phone and email, it's better to save that data in the user table so that is has the following structure:

Users ID_u name surename email phone

If you are set to keep the structure you have, my first tip is to take away the auto-increment on the table Application. That table is just meant to save the correlation between Users and InformationUser. Furthermore, the code you're looking for would be something like:

$sql = "SELECT * FROM Application WHERE ID_u = '1234'";
$result = mysql_query($sql) or die("Woops!");
if ($data = mysql_fetch_array($result)) {
 include("location1.php");
}
else {
 include("location2.php");
}

Answer 3

So you have users and you have applications. On each application there might be a different e-mail and name behind it. So you need to join and see if something exists. Something of this style

select count(*)
  from Application 
  join InformationUser on (InformationUser.ID = Application.ID)
 where 
       Application.ID_U = ?

The above gives you a count = 0 if both does not exist. If you want to know if both does not exists you can use an outer join.