Why navicat writes key instead of foreign key in CREATE TABLE?

CREATE TABLE IF NOT EXISTS `trl_bill` (
  `bill_id` int(11) NOT NULL AUTO_INCREMENT,
  `fee` double NOT NULL DEFAULT '0',
  `begin_date` date NOT NULL,
  `end_date` date NOT NULL,
  `is_paid` tinyint(4) NOT NULL DEFAULT '0',
  `request_id` int(11) NOT NULL,
  PRIMARY KEY (`bill_id`),
  KEY `requ开发者_运维问答est_id` (`request_id`) USING BTREE
) ENGINE=InnoDB DEFAULT CHARSET=utf8 ROW_FORMAT=FIXED AUTO_INCREMENT=1 ;

This is my table sql, i use navicat to design my database, why it writes KEY instead of FOREIGN KEY?

The KEY keyword is a synonym for INDEX in MySQL. You do not have a foreign key constraint specified in that CREATE TABLE command.

The CREATE TABLE syntax:

CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name
    { LIKE old_tbl_name | (LIKE old_tbl_name) }
create_definition:
    col_name column_definition
  | [CONSTRAINT [symbol]] PRIMARY KEY [index_type] (index_col_name,...)
      [index_option] ...
  | {INDEX|KEY} [index_name] [index_type] (index_col_name,...)
      [index_option] ...
  | [CONSTRAINT [symbol]] UNIQUE [INDEX|KEY]
      [index_name] [index_type] (index_col_name,...)
      [index_option] ...
  | {FULLTEXT|SPATIAL} [INDEX|KEY] [index_name] (index_col_name,...)
      [index_option] ...
  | [CONSTRAINT [symbol]] FOREIGN KEY
      [index_name] (index_col_name,...) reference_definition
  | CHECK (expr)