在MySQL中,FKs的"CONSTRAINT"是做什么的？

Question

我看过MySQL 5.1文档,但仍然无法弄清楚这一点 - 我注意到我输入MySQL的代码与系统输出代码之间存在差异.

代码示例01和02之间有什么区别,这意味着02 CONSTRAINT之前添加了FOREIGN KEY- 为什么,它有什么作用？

CODE_SAMPLE_01:

FOREIGN KEY (TABLE_02_nID_FK__TABLE_01_sID_PK) REFERENCES TABLE_01(TABLE_01_sID_PK),

背景:

CREATE TABLE `TABLE_02` (
  `TABLE_02_sID_PK` int(8) NOT NULL,
  `TABLE_02_nID_FK__TABLE_01_sID_PK` int(8) NOT NULL,
  `TABLE_02_cID` int(8) NOT NULL,
  `TABLE_02_data01` varchar(128) default NULL,
  `TABLE_02_data02` varchar(128) NOT NULL,
  `create_timestamp` DATETIME DEFAULT NULL,
  `update_timestamp` TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  PRIMARY KEY (`TABLE_02_sID_PK`),
  FOREIGN KEY (TABLE_02_nID_FK__TABLE_01_sID_PK) REFERENCES TABLE_01(TABLE_01_sID_PK),
  INDEX `TABLE_02_nID_FK__TABLE_01_sID_PK` (`TABLE_02_nID_FK__TABLE_01_sID_PK`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CODE_SAMPLE_02:

CONSTRAINT `table_02_ibfk_1` FOREIGN KEY (`TABLE_02_nID_FK__TABLE_01_sID_PK`) REFERENCES `table_01` (`TABLE_01_sID_PK`)

背景:

CREATE TABLE `table_02` (
  `TABLE_02_sID_PK` int(8) NOT NULL,
  `TABLE_02_nID_FK__TABLE_01_sID_PK` int(8) NOT NULL,
  `TABLE_02_cID` int(8) NOT NULL,
  `TABLE_02_data01` varchar(128) DEFAULT NULL,
  `TABLE_02_data02` varchar(128) NOT NULL,
  `create_timestamp` datetime DEFAULT NULL,
  `update_timestamp` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  PRIMARY KEY (`TABLE_02_sID_PK`),
  KEY `TABLE_02_nID_FK__TABLE_01_sID_PK` (`TABLE_02_nID_FK__TABLE_01_sID_PK`),
  CONSTRAINT `table_02_ibfk_1` FOREIGN KEY (`TABLE_02_nID_FK__TABLE_01_sID_PK`) REFERENCES `table_01` (`TABLE_01_sID_PK`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

Answer 1

可选的CONSTRAINT关键字允许您指定外键的名称.没有它,将自动生成名称.

可以在INFORMATION_SCHEMA TABLE_CONSTRAINTS表中看到此名称.