MySQL Cannot Add Foreign Key Constraint

To find the specific error run this:

SHOW ENGINE INNODB STATUS;

And look in the LATEST FOREIGN KEY ERROR section.

The data type for the child column must match the parent column exactly. For example, since medicalhistory.MedicalHistoryID is an INT, Patient.MedicalHistory also needs to be an INT, not a SMALLINT.

Also, you should run the query set foreign_key_checks=0 before running the DDL so you can create the tables in an arbitrary order rather than needing to create all parent tables before the relevant child tables.

I had set one field as "Unsigned" and other one not. Once I set both columns to Unsigned it worked.

Engine should be the same e.g. InnoDB

Datatype should be the same, and with same length. e.g. VARCHAR(20)

Collation Columns charset should be the same. e.g. utf8 Watchout: Even if your tables have same Collation, columns still could have different one.

Unique - Foreign key should refer to field that is unique (usually primary key) in the reference table.

Try to use the same type of your primary keys - int(11) - on the foreign keys - smallint(5) - as well.

Hope it helps!

Confirm that the character encoding and collation for the two tables is the same.

In my own case, one of the tables was using utf8 and the other was using latin1.

I had another case where the encoding was the same but the collation different. One utf8_general_ci the other utf8_unicode_ci

You can run this command to set the encoding and collation for a table.

ALTER TABLE tablename CONVERT TO CHARACTER SET utf8 COLLATE utf8_unicode_ci;

I hope this helps someone.

To set a FOREIGN KEY in Table B you must set a KEY in the table A.

In table A: INDEX id (id)

And then in the table B,

CONSTRAINT `FK_id` FOREIGN KEY (`id`) REFERENCES `table-A` (`id`)

I had same problem and the solution was very simple. Solution : foreign keys declared in table should not set to be not null.

reference : If you specify a SET NULL action, make sure that you have not declared the columns in the child table as NOT NULL. (ref )

Check following rules :

First checks whether names are given right for table names

Second right data type give to foreign key ?

Please ensure that both the tables are in InnoDB format. Even if one is in MyISAM format, then, foreign key constraint wont work.

Also, another thing is that, both the fields should be of the same type. If one is INT, then the other should also be INT. If one is VARCHAR, the other should also be VARCHAR, etc.

I faced the issue and was able to resolve it by making sure that the data types were exactly matching .

I was using SequelPro for adding the constraint and it was making the primary key as unsigned by default .

Check the signing on both your table columns. If the referring table column is SIGNED, the referenced table column should be SIGNED too.

My problem was that I was trying to create the relation table before other tables!

So you have two ways to fix it:

change the order of MSQL commands

run this before your queries: SET foreign_key_checks = 0;

NOTE: The following tables were taken from some site when I was doing some R&D on the database. So the naming convention is not proper.

For me, the problem was, my parent table had the different character set than that of the one which I was creating.

Parent Table (PRODUCTS)

products | CREATE TABLE `products` (
  `productCode` varchar(15) NOT NULL,
  `productName` varchar(70) NOT NULL,
  `productLine` varchar(50) NOT NULL,
  `productScale` varchar(10) NOT NULL,
  `productVendor` varchar(50) NOT NULL,
  `productDescription` text NOT NULL,
  `quantityInStock` smallint(6) NOT NULL,
  `buyPrice` decimal(10,2) NOT NULL,
  `msrp` decimal(10,2) NOT NULL,
  PRIMARY KEY (`productCode`),
  KEY `productLine` (`productLine`),
  CONSTRAINT `products_ibfk_1` FOREIGN KEY (`productLine`) REFERENCES `productlines` (`productLine`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1

Child Table which had a problem (PRICE_LOGS)

price_logs | CREATE TABLE `price_logs` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `productCode` varchar(15) DEFAULT NULL,
  `old_price` decimal(20,2) NOT NULL,
  `new_price` decimal(20,2) NOT NULL,
  `added_on` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`),
  KEY `productCode` (`productCode`),
  CONSTRAINT `price_logs_ibfk_1` FOREIGN KEY (`productCode`) REFERENCES `products` (`productCode`) ON DELETE CASCADE ON UPDATE CASCADE
);

MODIFIED TO

price_logs | CREATE TABLE `price_logs` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `productCode` varchar(15) DEFAULT NULL,
  `old_price` decimal(20,2) NOT NULL,
  `new_price` decimal(20,2) NOT NULL,
  `added_on` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`),
  KEY `productCode` (`productCode`),
  CONSTRAINT `price_logs_ibfk_1` FOREIGN KEY (`productCode`) REFERENCES `products` (`productCode`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=latin1 

One additional cause of this error is when your tables or columns contain reserved keywords:

Sometimes one does forget these.

For me the target table was blocking the foreign key. I had to set Auto-Increment (AI) on the table the Foreign-Key was pointing to.

I had a similar error in creating foreign key in a Many to Many table where the primary key consisted of 2 foreign keys and another normal column. I fixed the issue by correcting the referenced table name i.e. company, as shown in the corrected code below:

create table company_life_cycle__history -- (M-M)
(
company_life_cycle_id tinyint unsigned not null,
Foreign Key (company_life_cycle_id) references company_life_cycle(id) ON DELETE    CASCADE ON UPDATE CASCADE,
company_id MEDIUMINT unsigned not null,
Foreign Key (company_id) references company(id) ON DELETE CASCADE ON UPDATE CASCADE,
activity_on date NOT NULL,
PRIMARY KEY pk_company_life_cycle_history (company_life_cycle_id, company_id,activity_on),
created_on datetime DEFAULT NULL,
updated_on datetime DEFAULT NULL,
created_by varchar(50) DEFAULT NULL,
updated_by varchar(50) DEFAULT NULL
);

I had similar error with two foreign keys for different tables but with same key names! I have renamed keys and the error had gone)

Had a similar error, but in my case I was missing to declare the pk as auto_increment.

Just in case it could be helpful to anyone

I got the same error. The cause in my case was:

I created a backup of a database via phpmyadmin by copying the whole database. I created a new db with the same name the old db had und selected it. I started an SQL script to create updated tables and data. I got the error. Also when I disabled foreign_key_checks. Altough the database was completely empty.

The cause was: Since i used phpmyadmin to create some foreign keys in the renamed database - the foreign keys where created with a database name prefix but the database name prefix was not updated. So there were still references in the backup-db pointing to the newly created db.

My solution is maybe a little embarrassing and tells the tale of why you should sometimes look at what you have in front of you instead of these posts :)

I had ran a forward engineer before, which failed, so that meant that my database already had a few tables, then i have been sitting trying to fix foreign key contraints failures trying to make sure that everything was perfect, but it ran up against the tables previously created, so it was to no prevail.

In my case, there was a syntax error which was not explicitly notified by MySQL console upon running the query. However, SHOW ENGINE INNODB STATUS command's LATEST FOREIGN KEY ERROR section reported,

  Syntax error close to:

  REFERENCES`role`(`id`) ON DELETE CASCADE) ENGINE = InnoDB DEFAULT CHARSET = utf8

I had to leave a whitespace between REFERENCES and role to make it work.

For me it was - you can't omit prefixing the current DB table if you create a FK for a non-current DB referencing the current DB:

USE currrent_db;
ALTER TABLE other_db.tasks ADD CONSTRAINT tasks_fk FOREIGN KEY (user_id) REFERENCES currrent_db.users (id);

If I omit "currrent_db." for users table, I get the FK error. Interesting that SHOW ENGINE INNODB STATUS; shows nothing in this case.

My Solution!!

If we want to have column1 of table1 as a foreign key of table2, then column1 should be a key of table1.

For example, consider we have departments table, which has dept_id column.

Now let's say we have another table named employees which has emp_dept_id column.

If we want to use the dept_id column of the department table as a foreign key for the emp_dept_id column of emp, then the dept_id of department table SHOULD ATLEAST BE a key if not a primary key.

So make sure that dept_id of depratment is either a primary key or a unique key before using it as a foreign key for another table.

If you are getting this error with PhpMyAdmin, disable foreign key checks before importing the SQL file.

https://i.stack.imgur.com/B2GnA.png

I had this same issue then i corrected the Engine name as Innodb in both parent and child tables and corrected the reference field name FOREIGN KEY (c_id) REFERENCES x9o_parent_table(c_id)
then it works fine and the tables are installed correctly. This will be use full for someone.