如果value为空，则不查询SQL字符串

Question

I have two drop down menu's and when you select a value the value is saved in a session and passed trough AJAX to update a div. I have this working.

The reason I save the value in a session is because I need the database query to be dynamically filled (constructed). At this moment I have the following code:

    <?php
        $q = $_GET['q'];
        $_SESSION['theme'] = $q;
        $i = $_SESSION['category'];

        $query = "SELECT COUNT(".$q.") c FROM MirrorWebProductsExpanded WHERE Subcategorie = '".$i."'";
        $result = mysqli_query($conn,$query);
        $row = mysqli_fetch_assoc($result);
        echo "
        <a href='http://example.nl/search/'>" . $row['c'] .  "</a>";
    ?>

This counts all items from $q which is the value of the first drop down menu.

And WHERE Subcategorie = '".$i."', $i is the value from the second drop down menu.

But, now the problem. If the second value is empty (I haven't selected an option from that drop down menu) the query still add's this part to the query, like:

$query = "SELECT COUNT(".$q.") c FROM MirrorWebProductsExpanded WHERE Subcategorie = ''";.

This makes the count from the first drop down menu always show 0. Is there away to only add the WHERE Subcategorie = '".$i."' when the second drop down menu has a value?

I'm still kind of new to MySQL so please be nice...

Answer 1

Use a query without WHERE clause. Then check if $i is not null, empty or whitespace. If so, add WHERE clause to your query.

$query = "SELECT COUNT(".$q.") c FROM MirrorWebProductsExpanded";
if($i != "")
    $query = $query." WHERE Subcategorie = '".$i."'";