SELECT object_id FROM wp_term_relationships WHERE term_taxonomy_id = '7' AND term_taxonomy_id = '8'

SELECT DISTINCT object_id
FROM wp_term_relationships
WHERE term_taxonomy_id IN ('7', '8') AND object_id=1

$parametres = "'".$_GET['edat']."','".$_GET['pes']."','".$_GET['estatura']."'"; 
 
$sql = "SELECT DISTINCT wp.post_title, wp.ID FROM wp_posts wp, wp_term_relationships wtr WHERE wp.post_parent = '".$tipus[$i]."' AND wp.post_type = 'page' AND wp.ID = wtr.object_id AND wtr.term_taxonomy_id IN ($parametres)";
 
$sql_pagines_tipus = mysql_query($sql);
 
while ( $pagina = mysql_fetch_array($sql_pagines_tipus) ) { 
 
	if ( mostrarPagina($pagina['ID'], $_GET['edat'], $_GET['pes'], $_GET['estatura']) == true ) { ?>
		<li><?php echo $pagina['post_title'] ?><input type="checkbox" name="comparar[]" value="<?php echo $pagina['ID']; ?>"></li>
	<?php } 
}

function mostrarPagina($pagina_id, $edat, $pes, $estatura) 
{
 
    if ( $pes == '' ) { 
        $trobat_pes = true;
    }else{ 
        $trobat_pes = false; 
    }
 
    if ( $edat == '' ) { 
        $trobat_edat = true; 
    }else{ 
        $trobat_edat = false; 
    }
 
    if ( $estatura == '' ) { 
        $trobat_estatura = true;
    }else{ 
        $trobat_estatura = false; 
    }    
 
    $sql_tags = mysql_query("SELECT term_taxonomy_id FROM wp_term_relationships WHERE object_id = '$pagina_id'");
 
    while ( $row = mysql_fetch_array($sql_tags) ) {
 
        if ( $row[0] == $edat && $trobat_edat == false ) { $trobat_edat = true;}
        if ( $row[0] == $estatura && $trobat_estatura == false ) { $trobat_estatura = true; }
        if ( $row[0] == $pes && $trobat_pes == false ) { $trobat_pes = true; }
    }
 
 
    if ( $trobat_edat && $trobat_estatura && $trobat_pes) {
        return true;
    }else{
        return false;
    } 
}