protected function compileGroupBy(): string
	{
		if (! empty($this->QBGroupBy))
		{
			for ($i = 0, $c = count($this->QBGroupBy); $i < $c; $i ++)
			{
				// Is it already compiled?
				if (is_string($this->QBGroupBy[$i]))
				{
					continue;
				}

				$this->QBGroupBy[$i] = ($this->QBGroupBy[$i]['escape'] === false ||
						$this->isLiteral($this->QBGroupBy[$i]['field'])) ? $this->QBGroupBy[$i]['field'] : $this->db->protectIdentifiers($this->QBGroupBy[$i]['field']);
			}

			return "\nGROUP BY " . implode(', ', $this->QBGroupBy);
		}

		return '';
	}