Display terms from taxonomy B only if has relationship with a taxonomy term A

It should be possible by using the proper joins in the query that populates the second dropdown. Can you post the query you are currently using?

Hi vtxyzzy, thank you for replying, any help is appreciate. I’m not yet using a raw query, I’m just using a custom function in my functions.php. This function creates two dropdowns (city and cuisine terms) and populate (via ajax) the second dropdown with cuisine terms. The terms I didn’t translate, so ‘cidade’ means ‘city and ‘cozinha’ means ‘cuisine’:

function restaurant_search( $taxonomy ) {
	?>
	<script type="text/javascript">
		jQuery(document).ready(function( $ ) {
			$('#cidade').change(function(){
				$.ajax({
					url:"<?php bloginfo('wpurl'); ?>/wp-admin/admin-ajax.php",
					type:'POST',
					data:'action=category_select_action&name=' + name,
					success:function(results)
					{
					$("#cozinha").html(results);
					}
				 });
			});
		});
	</script>

<form action="<?php echo home_url('/'); ?>" role="search" method="get">
	<?php // First dropdown
		$term_selected_cidade = get_terms('cidade');
		echo '<select id="cidade" name="cidade">';
		echo '<option disabled="disabled" selected="selected">Cidade</option>';
		foreach ($term_selected_cidade as $term) {
			echo '<option value=' . $term->slug . '>' . $term->name . '</option>';
		}
		echo '</select>';
	?>

	<!-- Second Dropdown -->
	<select name="cozinha" id="cozinha" ><option>Escolha uma cidade</option></select>
	<input type="submit" value="buscar" />
</form>
<?php }

function implement_ajax() {
	// Populates the second dropdown with cuisine terms
	$term_selected_cozinha = get_terms('cozinha');
	echo '<select id="cozinha">';
	echo '<option disabled="disabled" selected="selected">Cozinha</option>';
	foreach ($term_selected_cozinha as $term) {
		echo '<option value=' . $term->slug . '>' . $term->name . '</option>';
	}
	echo '</select>';
}

add_action('wp_ajax_category_select_action', 'implement_ajax');
add_action('wp_ajax_nopriv_category_select_action', 'implement_ajax');

I’m trying to figure out the query, so I’ve got only half the way:

SELECT * FROM wp_terms AS wt
INNER JOIN wp_term_taxonomy AS wtt ON wt.term_id=wtt.term_id /* Juntando tabela de taxonomias */
INNER JOIN wp_term_relationships AS wtr ON wtr.term_taxonomy_id=wtt.term_taxonomy_id /* Juntando tabela de relationships */
LEFT JOIN wp_posts wp ON wp.ID=wtr.object_id
WHERE taxonomy='cidade' AND slug='recife'
ORDER BY name

I think I have set this up the way you need it, but I am not entirely sure.

The query is constructed of two parts. The ‘inner’ part selects the ID of all posts with the taxonomy of ‘cidade’ and the slug of the current city.

The ‘outer’ part then selects all of the terms in ‘cozinha’ which have a post whose ID is in the set selected by the ‘inner’ part.

SELECT DISTINCT wt.name
FROM wp_terms AS wt
INNER JOIN wp_term_taxonomy AS wtt ON wt.term_id=wtt.term_id /* Juntando tabela de taxonomias */
INNER JOIN wp_term_relationships AS wtr ON wtr.term_taxonomy_id=wtt.term_taxonomy_id /* Juntando tabela de relationships */
LEFT JOIN wp_posts wp ON wp.ID=wtr.object_id
WHERE taxonomy='cozinha'
AND wp.ID IN (
   SELECT wp.ID FROM wp_terms AS wt
   INNER JOIN wp_term_taxonomy AS wtt ON wt.term_id=wtt.term_id /* Juntando tabela de taxonomias */
   INNER JOIN wp_term_relationships AS wtr ON wtr.term_taxonomy_id=wtt.term_taxonomy_id /* Juntando tabela de relationships */
   LEFT JOIN wp_posts wp ON wp.ID=wtr.object_id
   WHERE taxonomy='cidade' AND slug='new-york'
)
ORDER BY name

Sorry for the delay in replying. Thank you very much indeed vtxyzzy! This is exactly what I was looking for, the query works like a charm!

Here is how the final code became in functions.php, if any kind of help for further reference:

function restaurant_search( $taxonomy ) {
	?>
	<script type="text/javascript">
		jQuery(document).ready(function() {
			$('#cidade').change(function(){
			var cidade=jQuery('#cidade').val();
				$.ajax({
					url:"<?php bloginfo('wpurl'); ?>/wp-admin/admin-ajax.php",
					type:'POST',
					data:'action=category_select_action&name=' + cidade,
					success:function(results)
					{
					$("#cozinha").html(results);
					}
				 });
			});
		});
	</script>

<form action="<?php echo home_url('/'); ?>" role="search" method="get">
	<?php // First dropdown
		$term_selected_cidade = get_terms('cidade');
		echo '<select id="cidade" name="cidade">';
		echo '<option disabled="disabled" selected="selected">Cidade</option>';
		foreach ($term_selected_cidade as $term) {
			echo '<option id=' . $term->term_id . ' value=' . $term->slug . '>' . $term->name . '</option>';
		}
		echo '</select>';

	?>

	<!-- Second Dropdown -->
	<select name="cozinha" id="cozinha" >
		<option>Escolha uma cidade</option>
	</select>
	<input type="submit" value="buscar" />
</form>

<?php }

function implement_ajax() {
	// Populates the second dropdown with cuisine terms
	global $wpdb;
	$cozinha_term = $_POST['name'];
	$query = "
		SELECT DISTINCT $wpdb->terms.name, $wpdb->terms.slug
		FROM $wpdb->terms
		INNER JOIN $wpdb->term_taxonomy ON $wpdb->terms.term_id=$wpdb->term_taxonomy.term_id
		INNER JOIN $wpdb->term_relationships ON $wpdb->term_relationships.term_taxonomy_id=$wpdb->term_taxonomy.term_taxonomy_id
		LEFT JOIN $wpdb->posts ON $wpdb->posts.ID=$wpdb->term_relationships.object_id
		WHERE $wpdb->term_taxonomy.taxonomy='cozinha' AND $wpdb->posts.ID IN (
		SELECT $wpdb->posts.ID
		FROM $wpdb->terms
		INNER JOIN $wpdb->term_taxonomy ON $wpdb->terms.term_id=$wpdb->term_taxonomy.term_id
		INNER JOIN $wpdb->term_relationships ON $wpdb->term_relationships.term_taxonomy_id=$wpdb->term_taxonomy.term_taxonomy_id
		LEFT JOIN $wpdb->posts ON $wpdb->posts.ID=$wpdb->term_relationships.object_id
		WHERE $wpdb->term_taxonomy.taxonomy='cidade' AND $wpdb->terms.slug='$cozinha_term')";

	$object = $wpdb->get_results($query);
	$cuisine_terms = array();
	for ($i = 0; $i < count($object); $i++) {
		$cuisine_terms[] = get_object_vars($object[$i]);
	}

	echo '<select id="cozinha">' . '<option disabled="disabled" selected="selected">Cozinha</option>';
	foreach ($cuisine_terms as $term) {
		echo '<option value=' . $term['slug'] . '>' . $term['name'] . '</option>';
	}
	echo '</select>';
}

add_action('wp_ajax_category_select_action', 'implement_ajax');
add_action('wp_ajax_nopriv_category_select_action', 'implement_ajax');