Mysql: Add Foreign Key to existing table

I want to add a Foreign Key to a table called "katalog".

ALTER TABLE katalog 
ADD CONSTRAINT `fk_katalog_sprache` 
FOREIGN KEY (`Sprache`)
REFERENCES `Sprache` (`ID`)
ON DELETE SET NULL
ON UPDATE SET NULL;

When I try to do this, I get this error message:

Error Code: 1005. Can't create table 'mytable.#sql-7fb1_7d3a' (errno: 150)

Error in INNODB Status:

120405 14:02:57 Error in foreign key constraint of table mytable.#sql-7fb1_7d3a:

FOREIGN KEY (`Sprache`)
REFERENCES `Sprache` (`ID`)
ON DELETE SET NULL
ON UPDATE SET NULL:
Cannot resolve table name close to:
(`ID`)
ON DELETE SET NULL
ON UPDATE SET NULL

When i use this query it works, but with wrong "on delete" action:

ALTER TABLE `katalog` 
ADD FOREIGN KEY (`Sprache` ) REFERENCES `sprache` (`ID` )

Both tables are InnoDB and both fields are "INT(11) not null". I'm using MySQL 5.1.61. Trying to fire this ALTER Query with MySQL Workbench (newest) on a MacBook Pro.

Table Create Statements:

CREATE TABLE `katalog` (
`ID` int(11) unsigned NOT NULL AUTO_INCREMENT,
`Name` varchar(50) COLLATE utf8_unicode_ci NOT NULL,
`AnzahlSeiten` int(4) unsigned NOT NULL,
`Sprache` int(11) NOT NULL,
PRIMARY KEY (`ID`),
UNIQUE KEY `katalogname_uq` (`Name`)
 ) ENGINE=InnoDB AUTO_INCREMENT=12 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci ROW_FORMAT=DYNAMIC$$

CREATE TABLE `sprache` (
`ID` int(11) NOT NULL AUTO_INCREMENT,
 `Bezeichnung` varchar(45) NOT NULL,
 PRIMARY KEY (`ID`),
 UNIQUE KEY `Bezeichnung_UNIQUE` (`Bezeichnung`),
KEY `ix_sprache_id` (`ID`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8

 Answers

To add a foreign key (grade_id) to an existing table (users), follow the following steps:

ALTER TABLE users ADD grade_id SMALLINT UNSIGNED NOT NULL DEFAULT 0;
ALTER TABLE users ADD CONSTRAINT fk_grade_id FOREIGN KEY (grade_id) REFERENCES grades(id);

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check this link. It has helped me with errno 150: http://verysimple.com/2006/10/22/mysql-error-number-1005-cant-create-table-mydbsql-328_45frm-errno-150/

On the top of my head two things come to mind.

• Is your foreign key index a unique name in the whole database (#3 in the list)?
• Are you trying to set the table PK to NULL on update (#5 in the list)?

I'm guessing the problem is with the set NULL on update (if my brains aren't on backwards today as they so often are...).

Edit: I missed the comments on your original post. Unsigned/not unsigned int columns maybe resolved your case. Hope my link helps someone in the future thought.

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Simple Steps...

ALTER TABLE t_name1 ADD FOREIGN KEY (column_name) REFERENCES t_name2(column_name)

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MySQL will execute this query:

ALTER TABLE `db`.`table1`
ADD COLUMN `col_table2_fk` INT UNSIGNED NULL,
ADD INDEX `col_table2_fk_idx` (`col_table2_fk` ASC),
ADD CONSTRAINT `col_table2_fk1`
FOREIGN KEY (`col_table2_fk`)
REFERENCES `db`.`table2` (`table2_id`)
ON DELETE NO ACTION
ON UPDATE NO ACTION;