Add Foreign Key to existing table

Question

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 

Answer 1

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);