Search multiple tables - leave table out if input is not filled in - php

Question

I have a page with 3 input fields, to search a database. But not all users are going to fill in all fields, so I need a way to make sure the database is checked fine.
Now, I wrote 8 different sql-statements, and with if-statements I check which fields are filled out. This works, but I do feel there must be a better to do this.
ID's in the search form are found in other tables in my database and loaded with jQuery's autocomplete.
Code now used:
<form action="" method="post">
<div class="ui-widget">
<section>
<label for="tags">Trefwoord:</label>
<input name="tags" id="tags"><input type="hidden" name="tags_id" class="tags_id" value="">
</section>
<section>
<label for="categorie">Categorie:</label>
<input name="cats" id="categorie"><input type="hidden" name="cats_id" class="cats_id" value="">
</section>
<section>
<label for="competentie">Competentie:</label>
<input name="com" id="competentie"><input type="hidden" name="com_id" class="com_id" value="">
</section>
<input type="submit" name="submit">
</div>
</form>
<?php
if(isset($_POST['submit'])) {
$trefwoord_id = $_POST['tags_id'];
$categorie_id = $_POST['cats_id'];
$p_trefwoord = $_POST['tags'];
$p_categorie = $_POST['cats'];
$competentie_id = $_POST['com_id'];
$p_comptentie = $_POST['com'];
if($trefwoord_id == null) {$sql = "SELECT * FROM spel_cat LEFT JOIN spel_com ON spel_cat.spelid = spel_com.spelid WHERE '$categorie_id' = catid && '$competentie_id' = comid";}
if($categorie_id == null) {$sql = "SELECT * FROM spel_tw LEFT JOIN spel_com ON spel_tw.spelid = spel_com.spelid WHERE '$trefwoord_id' = twid && '$competentie_id' = comid";}
if($competentie_id == null) {$sql = "SELECT * FROM spel_tw LEFT JOIN spel_cat ON spel_tw.spelid = spel_cat.spelid WHERE '$trefwoord_id' = twid && '$categorie_id' = catid";}
if($trefwoord_id == null && $categorie_id == null) {$sql = "SELECT * FROM spel_com WHERE '$competentie_id' = comid";}
if($trefwoord_id == null && $competentie_id == null) {$sql = "SELECT * FROM spel_cat WHERE '$categorie_id' = catid";}
if($categorie_id == null && $competentie_id == null) {$sql = "SELECT * FROM spel_tw WHERE '$trefwoord_id' = twid";}
if($trefwoord_id == null && $competentie_id == null && $categorie_id == null) {$sql = ""; echo "<b>Gebruik minstens 1 zoekterm</b>";}
if($trefwoord_id != null && $categorie_id != null && $competentie_id != null) {$sql = "SELECT * FROM (spel_tw LEFT JOIN spel_cat ON spel_tw.spelid = spel_cat.spelid) LEFT JOIN spel_com ON spel_cat.spelid = spel_com.spelid WHERE '$trefwoord_id' = twid && '$categorie_id' = catid && '$competentie_id' = comid";
}
if($sql != null) {
$games = mysqli_query($link,$sql) or die(mysql_error());
$num = mysqli_num_rows($games);
// AND SO ON...
WORKING CODE (thanks to OrangeHippo)
<?php
if(isset($_POST['submit'])) {
$trefwoord_id = $_POST['tags_id'];
$categorie_id = $_POST['cats_id'];
$p_trefwoord = $_POST['tags'];
$p_categorie = $_POST['cats'];
$competentie_id = $_POST['com_id'];
$p_comptentie = $_POST['com'];
$from = array();
$where = " 1 = 1 ";
if($trefwoord_id != null) {
$from["str"] = "spel_tw str";
$where .= " AND twid = '$trefwoord_id' ";
}
if($categorie_id != null) {
$from["sca"] = "spel_cat sca";
if (isset($from["str"])) {
$where .= " AND sca.spelid = str.spelid ";
}
$where .= " AND catid = '$categorie_id' ";
}
if($competentie_id != null) {
$from["sco"] = "spel_com sco";
if (isset($from["str"])) {
$where .= " AND sco.spelid = str.spelid ";
}else if (isset($from["sca"])) {
$where .= " AND sco.spelid = sca.spelid ";
}
$where .= " AND comid = '$competentie_id' ";
}
$sql = "SELECT * FROM " . implode(",", $from) . " WHERE $where";
if($trefwoord_id == null && $competentie_id == null && $categorie_id == null) {$sql = ""; echo "<b>Gebruik minstens 1 zoekterm</b>";}
//echo $sql;
if($sql != null) {
$games = mysqli_query($link,$sql) or die(mysql_error());
$num = mysqli_num_rows($games);
//AND SO ON ...

Answer 1

Ideally you would construct the query depending on the values that are passed:
$from = array();
$where = " 1 = 1 ";
if($trefwoord_id != null) {
$from["str"] = "spel_tw str";
$where .= " AND twid = '$trefwoord_id' ";
}
if($categorie_id != null) {
$from["sca"] = "spel_cat sca";
if (isset($from["str"])) {
$where = " AND sca.spelid = str.spelid ";
}
$where .= " AND catid = '$categorie_id' ";
}
if($competentie_id != null) {
$from["sco"] = "spel_com sco";
if (isset($from["str"])) {
$where = " AND sco.spelid = str.spelid ";
}else if (isset($from["sca"])) {
$where = " AND sco.spelid = sca.spelid ";
}
$where .= " AND comid = '$competentie_id' ";
}
$query = "SELECT * FROM " . implode(",", $from) . " WHERE $where";
As you see this way you have a lot less of text, making the code cleaner. If you want to have the code even more clean you can look to use some query builder library like doctrine2 DBAL