system/Database/Forge.php
protected function _alterTable(string $alter_type, string $table, $field)
	{
		$sql = 'ALTER TABLE ' . $this->db->escapeIdentifiers($table) . ' ';

		// DROP has everything it needs now.
		if ($alter_type === 'DROP')
		{
			return $sql . 'DROP COLUMN ' . $this->db->escapeIdentifiers($field);
		}

		$sql .= ($alter_type === 'ADD') ? 'ADD ' : $alter_type . ' COLUMN ';

		$sqls = [];
		foreach ($field as $data)
		{
			$sqls[] = $sql
					  . ($data['_literal'] !== false ? $data['_literal'] : $this->_processColumn($data));
		}

		return $sqls;
	}
protected function _alterTable(string $alter_type, string $table, $field)
	{
		$sql = 'ALTER TABLE ' . $this->db->escapeIdentifiers($table) . ' ';

		// DROP has everything it needs now.
		if ($alter_type === 'DROP')
		{
			return $sql . 'DROP COLUMN ' . $this->db->escapeIdentifiers($field);
		}

		$sql .= ($alter_type === 'ADD') ? 'ADD ' : $alter_type . ' COLUMN ';

		$sqls = [];
		foreach ($field as $data)
		{
			$sqls[] = $sql
					  . ($data['_literal'] !== false ? $data['_literal'] : $this->_processColumn($data));
		}

		return $sqls;
	}
ALTER TABLE

@param string $alter_type ALTER type
@param string $table      Table name
@param mixed  $field      Column definition

@return string|string[]