MySQL – Create Junction Table with Composite Key as Primary Key

mariadbMySQL

I have a category table which currently has only a title field (PK)

CREATE TABLE `category` (
  `title` varchar(20),
  PRIMARY KEY (`title`)
);

and a product table with a composite key as a primary key

CREATE TABLE `product` (
  `title` varchar(50),
  `vendor_name` varchar(50),
  `price` decimal(10,2) unsigned NOT NULL,
  PRIMARY KEY (`title`, `vendor_name`),
  CONSTRAINT `fk_product_vendor_name` FOREIGN KEY (`vendor_name`) REFERENCES `vendor` (`name`) ON DELETE CASCADE ON UPDATE CASCADE
);

A product can have many categories so I created a junction table. I'm struggling with the composite key from the product table. In the junction table I need to reference the combination of the product title and the vendor name foreign key. I started with this

CREATE TABLE `category_product` (
  ``/* product composite PK */
  `category_title` varchar(20),
  PRIMARY KEY (``/* product composite PK */,`category_title`),
  KEY `fk_category_product_category_title` (`category_title`),
  KEY ``/* product composite PK */
  CONSTRAINT `fk_category_product_category_title` FOREIGN KEY (`category_title`) REFERENCES `category` (`title`) ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT ``/* product composite PK */ ON DELETE CASCADE ON UPDATE CASCADE
);

How do I handle the composite key from the product table?

Thanks in advance

Best Answer

You have also to define two columns for a double Primary KEY

CREATE TABLE `category_product` (
  `category_title` varchar(20),
    `product_title` varchar(50),
  `product_vendor_name` varchar(50),
  CONSTRAINT `fk_category_product_category_title` FOREIGN KEY (`category_title`) REFERENCES `category` (`title`) ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `fk_category_product_product_title_vendor_name` FOREIGN KEY (`product_title`,`product_vendor_name`) REFERENCES `product` (`title`,`vendor_name`) ON DELETE CASCADE ON UPDATE CASCADE
);

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Mysql – “Cannot add or update a child row” when deleting a record

On the face of it, that does seem impossible.

The thing is, your error suggests it's not that you're trying to delete at all.

The message you're getting suggests you're trying to insert or update a row in the child table, not delete a row from the parent table. If the foreign key you posted was causing the problem relative to a delete, you should see this message, instead:

Cannot delete or update a parent row: a foreign key constraint fails (`test`.`bar`, CONSTRAINT `bar_ibfk_1` FOREIGN KEY (`foo_id`) REFERENCES `foo` (`id`))

It's also possible you have some BEFORE DELETE trigger magic on survey_main that's doing something unexpected.

Right after this error occurs, try this:

SHOW ENGINE INNODB STATUS;

The LATEST FOREIGN KEY ERROR section should give you something more to go on. Failing that, you could enable the general log, which will show queries executed by triggers and other stored programs, as well as the queries you're directly executing, to shed light on what might be going on behind the scenes.

Update (#1) Things are definitely not as they seem and the full table definitions are going to be pretty critical, here.

Also, the version of MySQL you're using may also be relevant, so please mention it.

With nothing more to go on at the moment, I'm speculating that you have invalid data in the survey_id column of the survey_answers table. To test that theory:

SELECT *
  FROM survey_answers sa
  LEFT JOIN survey_main sm ON sm.id = sa.survey_id
 WHERE sm.id IS NULL;

If I understand your schema correctly, then this query will return zero rows if I am wrong. :) If you get rows returned, then those rows have survey_answers records that contain an survey_id value that doesn't exist in the id column of survey_main.

Mysql – Finding rows for a specified date range

Another way to get the result is this. It finds first all groups that the teacher has surely taught (or is going to) by checking that she has started within the month and then in another subquery it finds - for every group - the last teacher that started at the first day of the month or earlier.

With the unique index you have on the table, the second subquery should be quite efficient. The first subquery would benefit from an index on (teacherid, startdate, groupid):

SELECT groupid
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UNION DISTINCT

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