Source

File: wp-includes/class-wp-tax-query.php

public function get_sql_for_clause( &$clause, $parent_query ) {
		global $wpdb;

		$sql = array(
			'where' => array(),
			'join'  => array(),
		);

		$join  = '';
		$where = '';

		$this->clean_query( $clause );

		if ( is_wp_error( $clause ) ) {
			return self::$no_results;
		}

		$terms    = $clause['terms'];
		$operator = strtoupper( $clause['operator'] );

		if ( 'IN' === $operator ) {

			if ( empty( $terms ) ) {
				return self::$no_results;
			}

			$terms = implode( ',', $terms );

			/*
			 * Before creating another table join, see if this clause has a
			 * sibling with an existing join that can be shared.
			 */
			$alias = $this->find_compatible_table_alias( $clause, $parent_query );
			if ( false === $alias ) {
				$i     = count( $this->table_aliases );
				$alias = $i ? 'tt' . $i : $wpdb->term_relationships;

				// Store the alias as part of a flat array to build future iterators.
				$this->table_aliases[] = $alias;

				// Store the alias with this clause, so later siblings can use it.
				$clause['alias'] = $alias;

				$join .= " LEFT JOIN $wpdb->term_relationships";
				$join .= $i ? " AS $alias" : '';
				$join .= " ON ($this->primary_table.$this->primary_id_column = $alias.object_id)";
			}

			$where = "$alias.term_taxonomy_id $operator ($terms)";

		} elseif ( 'NOT IN' === $operator ) {

			if ( empty( $terms ) ) {
				return $sql;
			}

			$terms = implode( ',', $terms );

			$where = "$this->primary_table.$this->primary_id_column NOT IN (
				SELECT object_id
				FROM $wpdb->term_relationships
				WHERE term_taxonomy_id IN ($terms)
			)";

		} elseif ( 'AND' === $operator ) {

			if ( empty( $terms ) ) {
				return $sql;
			}

			$num_terms = count( $terms );

			$terms = implode( ',', $terms );

			$where = "(
				SELECT COUNT(1)
				FROM $wpdb->term_relationships
				WHERE term_taxonomy_id IN ($terms)
				AND object_id = $this->primary_table.$this->primary_id_column
			) = $num_terms";

		} elseif ( 'NOT EXISTS' === $operator || 'EXISTS' === $operator ) {

			$where = $wpdb->prepare(
				"$operator (
				SELECT 1
				FROM $wpdb->term_relationships
				INNER JOIN $wpdb->term_taxonomy
				ON $wpdb->term_taxonomy.term_taxonomy_id = $wpdb->term_relationships.term_taxonomy_id
				WHERE $wpdb->term_taxonomy.taxonomy = %s
				AND $wpdb->term_relationships.object_id = $this->primary_table.$this->primary_id_column
			)",
				$clause['taxonomy']
			);

		}

		$sql['join'][]  = $join;
		$sql['where'][] = $where;
		return $sql;
	}