Ungisned - One table could have key identified as unsigned and other is not.Unique - Foreign key should refer to field that is unique (usually primary key) in the reference table.Watchout: Even if your tables have same Collation, columns still could have different one.Collation Columns charset should be the same.int(11) on one table and smallint(5) on another will cause problem. Datatype both columns should have same datatype.Engine both tables should have same engine specfied e.g.It can also be because of different collation on the tables, one UTF-8 and the other latin1įollowing are the things you should look for It can also be a mismatch in the Engine type of two tables i.e. The usual cause are generally a mismatch in the type of the column of the primary table and the foreign table. In here you will find the exact cause of why the last alter query failed and then you can proceed to fix it. Expand the status column of the result and look for LATEST FOREIGN KEY ERROR. This query will return three fields Type, Name and Status. To fix this issue you need to get to know the exact cause, and to find that run the below query in your mysql client SHOW ENGINE INNODB STATUS This error is very general and does not show the specific cause of why MySQL cannot add a foreign key constraint to the table. The length of string types need not be the same.If you are altering a table in your MySQL to add foreign key constraint, and receiving the following errorĮRROR 1215 (HY000): Cannot add foreign key constraint The size and sign of integer types must be The cause of our error was due to violation of following rule:Ĭorresponding columns in the foreign key and the referenced key must So if the index in referenced table is exist and it is consists from several columns, and desired column is not first, the error shall be occurred. A foreign key constraint is a rule that enforces the relationship between two tables. In this case, we are adding a foreign key constraint to the Movie table. Where the referenced columns are listed as the first columns in the ALTER TABLE Movie ADD FOREIGN KEY (Year) REFERENCES YearStats (Year) ON DELETE NO ACTION The ALTER TABLE statement is used to modify the structure of an existing table. However, in the referenced table, there must be an index InnoDB permits a foreign key to reference any index column or group ofĬolumns. Referencing table, there must be an index where the foreign keyĬolumns are listed as the first columns in the same order. MySQL requires indexes on foreign keys and referenced keys so thatįoreign key checks can be fast and not require a table scan. Does anyone have any suggestions? And what is meant by first columns? So I guess the problem must be thatĬannot find an index in the referenced table where the referenced columns appear as the first columns.īut I'm not quite sure what this means, or how to check/fix it. (I also tried setting the experiment column to not null, but this didn't fix it). To me, the column types seem to match, since they are both varchar(45). In the table and the referenced table do not match for constraint. Referenced columns appear as the first columns, or column types To get some more information, I did: SHOW ENGINE INNODB STATUS\Gįrom which I got: FOREIGN KEY (experiment) REFERENCES exp (EID)Ĭannot find an index in the referenced table where the Insert_date timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,ĬONSTRAINT FOREIGN KEY (inserted_by) REFERENCES user (iduser),ĬONSTRAINT FOREIGN KEY (target) REFERENCES protein (PID),ĬONSTRAINT FOREIGN KEY (sample_type) REFERENCES sample_type (ID),ĬONSTRAINT FOREIGN KEY (assay) REFERENCES assays (AID),ĬONSTRAINT FOREIGN KEY (experiment) REFERENCES exp (EID)ĮRROR 1215 (HY000): Cannot add foreign key constraint Sample_type mediumint(5) UNSIGNED NOT NULL,Īssay mediumint(5) UNSIGNED ZEROFILL NOT NULL, ![]() | inserted_by | int(11) unsigned | YES | MUL | NULL | |Īnd I wan't to create a new table called sample_df referencing this, using the following: CREATE TABLE sample_df (ĭf_id mediumint(5) UNSIGNED AUTO_INCREMENT PRIMARY KEY, | Initials | varchar(255) | NO | | NULL | | ![]() | EID | varchar(45) | NO | PRI | NULL | | | Field | Type | Null | Key | Default | Extra | I'm having some problems creating a foreign key to an existing table in a MySQL database.
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