外键列的唯一约束

Question

I'm trying to add a unique constraint to two foreign keys:

CREATE TABLE tagsInBlog(
    id_tag int(10) not null auto_increment,
    id_word int(10) not null,
    id_page int(11),
    PRIMARY KEY(id_tag),
    FOREIGN KEY (id_page) REFERENCES archive(id),
    FOREIGN KEY (id_word) REFERENCES tagwords(id_word)
)ENGINE=INNODB DEFAULT CHARSET=utf8;

ALTER TABLE tagsinblog
  ADD UNIQUE tagBlogConstraint (id_word, id_page);

When creating I don't get any errors, but when I'm trying to insert I get:

MYSQL ERROR 367421 (could not save new tag data into Mysql): Error 1452 (23000): Cannot add or update a child row: a foreign key constraint fails ( sqse_001 . tagsinblog , CONSTRAINT tagsinblog_ibfk_2 FOREIGN KEY ( id_word ) REFERENCES tagwords ( id_word ))

When I'm trying to insert in the same table without the unique constraint I don't have any problems.

Answer 1

I worked on your problem statement and assumed few things as follow

your archive table may looks like this

CREATE TABLE IF NOT EXISTS `archive` ( `id` int(11) NOT NULL AUTO_INCREMENT,
`descrp` varchar(10) NOT NULL DEFAULT '', PRIMARY KEY (id) ) ENGINE=InnoDB

tagwords table may looks like this

 CREATE TABLE IF NOT EXISTS `tagwords` ( `id_word` int(11) NOT NULL
 AUTO_INCREMENT, `descrp` varchar(10) NOT NULL DEFAULT '', 
 PRIMARY KEY (id_word) ) ENGINE=InnoDB

Now your query for table tagsInBlog

CREATE TABLE tagsInBlog(
id_tag int(10) not null auto_increment,
id_word int(10) not null,
id_page int(11),
PRIMARY KEY(id_tag),
FOREIGN KEY (id_page) REFERENCES archive(id),
FOREIGN KEY (id_word) REFERENCES tagwords(id_word)
)ENGINE=INNODB DEFAULT CHARSET=utf8; 

Alter Query for table tagsInBlog

ALTER TABLE tagsinblog ADD UNIQUE tagBlogConstraint (id_word, id_page);

Following Insert statements works fine

INSERT INTO `test`.`tagsinblog` (`id_tag`, `id_word`, `id_page`) 
VALUES (NULL, '1', '1'), (NULL, '1', '2');

assuming you have respective entry in table tagswords and archive

But if your try to insert any value as foreign key which value doesn't exist in tables archive or tagwords , then it will throw following error

 #1452 - Cannot add or update a child row: a foreign key constraint fails  
 (`test`.`tagsinblog`, CONSTRAINT `tagsinblog_ibfk_2` 
 FOREIGN KEY (`id_word`) REFERENCES `tagwords` (`id_word`)) 

So Make sure you have proper entry in all table.

Hope it helps!