These relationships are managed through the use of foreign keys, essentially, fields that have the same meaning in all the tables in the relationship and that serve as points of commonality to link records in different tables together. By making it possible to easily relate records in different tables to one another, an RDBMS makes it possible to analyze data in different ways while simultaneously keeping it organized in a systematic fashion, with minimal redundancy. The fundamental basis of a relational database system like MySQL is its capability to create relationships between the tables that make up the database. When designing a set of database tables, it is important to specify which fields will be used for primary and foreign keys to clarify both in-table structure and inter-table relationships. The length of string types need not be the same.Primary keys serve as unique identifiers for the records in a table, while foreign keys are used to link related tables together. 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. Where the referenced columns are listed as the first columns in the 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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