如何使用通过HTML表单发送的变量显示mysql中的某些行

If we shorten your if (isset($_POST ... something you can clearly see. This instruction

$results = $conn->query($query);

is always executed, regardless of whether isset returns true or not.

if (isset($_POST['Kohderyhmä']) &&
 ...)
{
  $Kohderyhmä = get_post($conn, 'Kohderyhmä');
  ...
  $query = "SELECT * FROM `tietokanta`...."
}

$results = $conn->query($query);

So if only one field has not been filled out correctly, the error is always the same :
PHP Warning: mysqli::query(): Empty query in ....
This makes it difficult to determine where the fault really comes from.

Place the curly bracket } behind database logic.

if (isset($_POST['Kohderyhmä']) &&
 ...)
{
  $Kohderyhmä = get_post($conn, 'Kohderyhmä');
  ...
  $query = "SELECT * FROM `tietokanta`...."
  $results = $conn->query($query);
  if (!$results) die ("Database access failed: " . $conn->error);
  $rows = $results->num_rows;
  for ($j = 0 ; $j < $rows ; ++$j)
   {
     $results->data_seek($j);
     $row = $results->fetch_array(MYSQLI_ASSOC);
     ....
   } 
}
?>

create a short test program to test only the database. Set only really necessary data fields in the query

test.php

<?php
$conn = new mysqli('localhost', 'user', 'pass', 'db');
if ($conn->connect_error) die($conn->connect_error);
$conn->set_charset("utf8");

$Kohderyhmä = "KohderyTest"; // replace with really existing values

$query = "SELECT * FROM `tietokanta` WHERE Kohderyhmä='".$Kohderyhmä."' ";
$results = $conn->query($query);
if (!$results) die ("Database access failed: " . $conn->error);

  while ($row = $results->fetch_assoc()) {
    echo "<h3>" . $row['Nimi'] . "</h3><br />";
    echo $row['Kohderyhmä'] ."<br /><br />";
  }
$results->free();
?>

Add hardcoded variables $Näytön_aste = "reallyExistingValue"; , add query data field for data field and watch when it starts to stutter.

---

Also we can not see your function get_post()

If you mean the Wordpress function get_post(), your call to the function is wrong.

I can well imagine that the failure from the function get_post() comes.
And you always false or empty values assigns.

$Kohderyhmä = get_post($conn, 'Kohderyhmä');

assign it direct.

$post = $_POST;
if (isset($post['Kohderyhmä']) &&
...)
{
$Kohderyhmä = $post['Kohderyhmä'];
...

---

Also you are using all select fields from the <form>, in the query.

4 Select's with 8,6,6,6 options means

8x6x6x6 == 1728

1728 possibilities are you shure you have one datarecord where all values matches.

WHERE Ko...='$Ko...' AND `Näy...`='$Näy...' AND `Vai...`='$Vai...' AND `Käy...`='$Käy...'";

WHERE All four Datafields must match to get a result !!!!!!!!!!!!!

You have to find a combination where all four values simultaneously exist.

---

UPDATE

OP new question :
If you want empty or some named values stop searching for a value in database.

required every single variable to be found in the database which it didn't find because I couldn't set the variable and there is no value for "Kaikki" in the database, the word "Kaikki" means all choices below that choice in the HTML form and for that I need some PHP

Here comes the new test.php

• 1) don't do $post['Näytön aste']; In the form the name is
  <select name="Näytön aste" style="...">.
  This will translated by submit to
  $post['Näytön_aste']; look at the underscore _
  This must be done with all select name with spaces in the name !!
• 2) That was the reason why you get not all $_POST[....] values !
  OK ?
• 3) replace in your form
  <form action="<?php $_SERVER['PHP_SELF']; ?>" method="POST">
  with
  <form action="testNew.php" method="POST">

---

testNew.php

---

<?php
$conn = new mysqli('localhost', 'user', 'pass', 'db');
if ($conn->connect_error) die($conn->connect_error);
$conn->set_charset("utf8");
$post = $_POST;
if (isset($post['Kohderyhmä']) &&
    isset($post['Näytön_aste']) &&
    isset($post['Vaikutusten_vahvuus']) &&
    isset($post['Käyttökelpoisuus']))
 {
  $Kohderyhmä = $post['Kohderyhmä'];
  $Näytön_aste = $post['Näytön_aste'];
  $Vaikutusten_vahvuus = $post['Vaikutusten_vahvuus'];
  $Käyttökelpoisuus = $post['Käyttökelpoisuus'];
 } else { die ("No valid values"); }
    $count = 0;
    $and   = "";
    $query = "";

    if (!empty($Kohderyhmä) && $Kohderyhmä !="Kaikki" ) {
      if ($count > 0) { $and   = " AND ";  }
      $count++;
      $query = $query.$and."`Kohderyhmä`= '".$Kohderyhmä."'";
    }
    if (!empty($Näytön_aste) && $Näytön_aste !="Kaikki" ) {
      if ($count > 0) { $and   = " AND ";  }
      $count++;
      $query = $query.$and."`Näytön aste`= '".$Näytön_aste."'";
    }
    if (!empty($Vaikutusten_vahvuus) && $Vaikutusten_vahvuus !="Kaikki" ) {
      if ($count > 0) { $and   = " AND ";  }
      $count++;
      $query = $query.$and."`Vaikutusten vahvuus`= '".$Vaikutusten_vahvuus."'";
    }
    if (!empty($Käyttökelpoisuus) && $Käyttökelpoisuus !="Kaikki" ) {
      if ($count > 0) { $and   = " AND ";  }
      $count++;
      $query = $query.$and."`Käyttökelpoisuus: luokka`= '".$Käyttökelpoisuus."'";
    }
    if ($count > 0) {
        $query = "SELECT * FROM `tietokanta` WHERE ".$query;
        } else {
        $query = "SELECT * FROM `tietokanta`";    
        } 
    echo $query;

if ($results = $conn->query($query)) {
while ($row = $results->fetch_assoc()) {
    echo "<h3>" . $row['Nimi'] . "</h3><br />";
    echo $row['Kohderyhmä'] ."<br /><br />";
  }
} else {
  echo "with your choices no records were found";
}
$results->free();
?>

$query = "SELECT * FROM `tietokanta`
    WHERE Kohderyhmä='{$Kohderyhmä}' AND `Näytön aste`='{$Näytön_aste}' AND `Vaikutusten vahvuus`='{$Vaikutusten_vahvuus}' AND `Käyttökelpoisuus: luokka`='{$Käyttökelpoisuus}'";

Replace your query with this, and try.