<?php

function wp_tax_and() {
	$args = func_get_args();

	return wp_tax_group('AND', $args);
}

function wp_tax_or() {
	$args = func_get_args();

	return wp_tax_group('OR', $args);
}

function wp_tax_group($op, $args) {
	return "(" . implode(" $op ", $args) . ")";
}

function wp_tax($taxonomy, $terms, $field = 'term_id') {
	return _wp_tax($taxonomy, $terms, $field, true);
}

function wp_tax_not($taxonomy, $terms, $field = 'term_id') {
	return _wp_tax($taxonomy, $terms, $field, false);
}

function _wp_tax($taxonomy, $terms, $field, $in) {
	global $wpdb;

	$terms = (array) $terms;

	switch ( $field ) {
		case 'term_taxonomy_id':
			$terms = implode( ',', array_map('intval', $terms) );
		break;

		case 'term_id':
			$terms = implode( ',', array_map('intval', $terms) );
			$terms = $wpdb->prepare("
				SELECT term_taxonomy_id
				FROM $wpdb->term_taxonomy
				WHERE taxonomy = %s
				AND term_id IN ($terms)
			", $taxonomy);
		break;

		case 'slug':
		case 'name':
			$terms = "'" . implode( "','", esc_sql($terms) ) . "'";
			$terms = $wpdb->prepare("
				SELECT term_taxonomy_id
				FROM $wpdb->term_taxonomy
				INNER JOIN $wpdb->terms USING (term_id)
				WHERE taxonomy = %s
				AND $field IN ($terms)
			", $taxonomy);
	}

	$operator = $in ? 'IN' : 'NOT IN';

	return "object_id IN (
		SELECT object_id FROM $wpdb->term_relationships WHERE term_taxonomy_id $operator ($terms)
	)";
}

function wp_tax_query($query) {
	global $wpdb;
	return "SELECT DISTINCT object_id FROM $wpdb->term_relationships WHERE " . $query;
}


// Example
$query = wp_tax_query(wp_tax_and(
	wp_tax_or(
		wp_tax( 'post_category', 59 ),
		wp_tax( 'post_tag', 'web', 'slug' )
	),
	wp_tax_not( 'post_category', 24 ),
	wp_tax_not( 'post_tag', 'wordpress', 'slug' )
));

var_dump($query);