MySQL Foreign Key – Preventing Deletion of Empty Table

foreign keyMySQL

Why would dropping an empty table produce a foreign key error?

In MySQL, I have a table with the following definition

CREATE TABLE `enterprise_rma` (
  `entity_id` int(10) unsigned NOT NULL AUTO_INCREMENT COMMENT 'RMA Id',
  `status` varchar(32) DEFAULT NULL COMMENT 'Status',
  `is_active` smallint(5) unsigned NOT NULL DEFAULT '1' COMMENT 'Is Active',
  `increment_id` varchar(50) DEFAULT NULL COMMENT 'Increment Id',
  `date_requested` timestamp NULL DEFAULT CURRENT_TIMESTAMP COMMENT 'RMA Requested At',
  `order_id` int(10) unsigned NOT NULL COMMENT 'Order Id',
  `order_increment_id` varchar(50) DEFAULT NULL COMMENT 'Order Increment Id',
  `store_id` smallint(5) unsigned DEFAULT NULL COMMENT 'Store Id',
  `customer_id` int(10) unsigned DEFAULT NULL,
  `customer_custom_email` varchar(255) DEFAULT NULL COMMENT 'Customer Custom Email',
  PRIMARY KEY (`entity_id`),
  KEY `IDX_ENTERPRISE_RMA_STATUS` (`status`),
  KEY `IDX_ENTERPRISE_RMA_IS_ACTIVE` (`is_active`),
  KEY `IDX_ENTERPRISE_RMA_INCREMENT_ID` (`increment_id`),
  KEY `IDX_ENTERPRISE_RMA_DATE_REQUESTED` (`date_requested`),
  KEY `IDX_ENTERPRISE_RMA_ORDER_ID` (`order_id`),
  KEY `IDX_ENTERPRISE_RMA_ORDER_INCREMENT_ID` (`order_increment_id`),
  KEY `IDX_ENTERPRISE_RMA_STORE_ID` (`store_id`),
  KEY `IDX_ENTERPRISE_RMA_CUSTOMER_ID` (`customer_id`),
  CONSTRAINT `FK_ENTERPRISE_RMA_CUSTOMER_ID_CUSTOMER_ENTITY_ENTITY_ID` FOREIGN KEY (`customer_id`) REFERENCES `customer_entity` (`entity_id`) ON DELETE SET NULL ON UPDATE CASCADE,
  CONSTRAINT `FK_ENTERPRISE_RMA_STORE_ID_CORE_STORE_STORE_ID` FOREIGN KEY (`store_id`) REFERENCES `core_store` (`store_id`) ON DELETE SET NULL ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='RMA LIst';

I'm trying to DROP this table, but whenever I do I receive the following error.

Cannot delete or update a parent row: a foreign key constraint fails

What I don't understand is, the table is already empty. If I SELECT * FROM enterprise_rma;, no rows are returned (0 rows affected)

enter image description here

and SHOW ENGINE INNODB STATUS; returns nothing useful.

I know I can work around this with the following

SET FOREIGN_KEY_CHECKS=0; 
DROP TABLE enterprise_rma;
SET FOREIGN_KEY_CHECKS=0;

But I don't understand how a foreign key constraint could fail if there's no data in the table. Is there a subtlety of MySQL's foreign key mode I don't understand? Can MySQL get damaged or put in an invalid state when this happens? Are there methods for troubleshooting this myself?

enter image description here

Best Answer

You shouldn't be able to drop a table that is the parent of another dependent table, even if it has no data. Because if you drop the parent, then you will never be able to insert any data in the child. You must first drop any child tables that reference enterprise_rma.

Something doesn't match your description, though. You said you were having trouble dropping the table, but your example about the foreign key checks shows you using DELETE.

DELETE is not the same as DROP TABLE. After DELETE, there are no rows in the table, but the table still exists. After DROP TABLE, the table doesn't exist, i.e. it doesn't show up if you run SHOW TABLES.

Related Solutions

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
FROM pupilgroupteacher
WHERE teacherid = @teacher 
  AND startdate >= @month + INTERVAL 1 DAY
  AND startdate < @month + INTERVAL 1 MONTH

UNION DISTINCT

SELECT gg.groupid
FROM 
    ( SELECT DISTINCT groupid
      FROM pupilgroupteacher
    ) AS gd
  JOIN pupilgroupteacher AS gg
    ON  gg.groupid = gd.groupid
    AND gg.startdate =
        ( SELECT MAX(gi.startdate)
          FROM pupilgroupteacher AS gi 
          WHERE gi.groupid = gd.groupid
            AND gi.startdate < @month + INTERVAL 1 DAY
        )
WHERE gg.teacherid = @teacher ;

Best Answer

Related Solutions

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

Mysql – Finding rows for a specified date range

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