system/Database/BaseBuilder.php
public function join(string $table, string $cond, string $type = '', bool $escape = null)
	{
		if ($type !== '')
		{
			$type = strtoupper(trim($type));

			if (! in_array($type, ['LEFT', 'RIGHT', 'OUTER', 'INNER', 'LEFT OUTER', 'RIGHT OUTER'], true))
			{
				$type = '';
			}
			else
			{
				$type .= ' ';
			}
		}

		// Extract any aliases that might exist. We use this information
		// in the protectIdentifiers to know whether to add a table prefix
		$this->trackAliases($table);

		is_bool($escape) || $escape = $this->db->protectIdentifiers;

		if (! $this->hasOperator($cond))
		{
			$cond = ' USING (' . ($escape ? $this->db->escapeIdentifiers($cond) : $cond) . ')';
		}
		elseif ($escape === false)
		{
			$cond = ' ON ' . $cond;
		}
		else
		{
			// Split multiple conditions
			if (preg_match_all('/\sAND\s|\sOR\s/i', $cond, $joints, PREG_OFFSET_CAPTURE))
			{
				$conditions = [];
				$joints     = $joints[0];
				array_unshift($joints, ['', 0]);

				for ($i = count($joints) - 1, $pos = strlen($cond); $i >= 0; $i --)
				{
					$joints[$i][1] += strlen($joints[$i][0]); // offset
					$conditions[$i] = substr($cond, $joints[$i][1], $pos - $joints[$i][1]);
					$pos            = $joints[$i][1] - strlen($joints[$i][0]);
					$joints[$i]     = $joints[$i][0];
				}
			}
			else
			{
				$conditions = [$cond];
				$joints     = [''];
			}

			$cond = ' ON ';
			for ($i = 0, $c = count($conditions); $i < $c; $i ++)
			{
				$operator = $this->getOperator($conditions[$i]);
				$cond    .= $joints[$i];
				$cond    .= preg_match("/(\(*)?([\[\]\w\.'-]+)" . preg_quote($operator) . '(.*)/i', $conditions[$i], $match) ? $match[1] . $this->db->protectIdentifiers($match[2]) . $operator . $this->db->protectIdentifiers($match[3]) : $conditions[$i];
			}
		}

		// Do we want to escape the table name?
		if ($escape === true)
		{
			$table = $this->db->protectIdentifiers($table, true, null, false);
		}

		// Assemble the JOIN statement
		$this->QBJoin[] = $join = $type . 'JOIN ' . $table . $cond;

		return $this;
	}
public function join(string $table, string $cond, string $type = '', bool $escape = null)
	{
		if ($type !== '')
		{
			$type = strtoupper(trim($type));

			if (! in_array($type, ['LEFT', 'RIGHT', 'OUTER', 'INNER', 'LEFT OUTER', 'RIGHT OUTER'], true))
			{
				$type = '';
			}
			else
			{
				$type .= ' ';
			}
		}

		// Extract any aliases that might exist. We use this information
		// in the protectIdentifiers to know whether to add a table prefix
		$this->trackAliases($table);

		is_bool($escape) || $escape = $this->db->protectIdentifiers;

		if (! $this->hasOperator($cond))
		{
			$cond = ' USING (' . ($escape ? $this->db->escapeIdentifiers($cond) : $cond) . ')';
		}
		elseif ($escape === false)
		{
			$cond = ' ON ' . $cond;
		}
		else
		{
			// Split multiple conditions
			if (preg_match_all('/\sAND\s|\sOR\s/i', $cond, $joints, PREG_OFFSET_CAPTURE))
			{
				$conditions = [];
				$joints     = $joints[0];
				array_unshift($joints, ['', 0]);

				for ($i = count($joints) - 1, $pos = strlen($cond); $i >= 0; $i --)
				{
					$joints[$i][1] += strlen($joints[$i][0]); // offset
					$conditions[$i] = substr($cond, $joints[$i][1], $pos - $joints[$i][1]);
					$pos            = $joints[$i][1] - strlen($joints[$i][0]);
					$joints[$i]     = $joints[$i][0];
				}
			}
			else
			{
				$conditions = [$cond];
				$joints     = [''];
			}

			$cond = ' ON ';
			for ($i = 0, $c = count($conditions); $i < $c; $i ++)
			{
				$operator = $this->getOperator($conditions[$i]);
				$cond    .= $joints[$i];
				$cond    .= preg_match("/(\(*)?([\[\]\w\.'-]+)" . preg_quote($operator) . '(.*)/i', $conditions[$i], $match) ? $match[1] . $this->db->protectIdentifiers($match[2]) . $operator . $this->db->protectIdentifiers($match[3]) : $conditions[$i];
			}
		}

		// Do we want to escape the table name?
		if ($escape === true)
		{
			$table = $this->db->protectIdentifiers($table, true, null, false);
		}

		// Assemble the JOIN statement
		$this->QBJoin[] = $join = $type . 'JOIN ' . $table . $cond;

		return $this;
	}
JOIN

Generates the JOIN portion of the query

@param string    $table
@param string    $cond   The join condition
@param string    $type   The type of join
@param boolean   $escape Whether not to try to escape identifiers

@return BaseBuilder