[mysql] ERROR 1452: Cannot add or update a child row: a foreign key constraint fails

I have created tables in MySQL Workbench as shown below :

ORDRE table:

CREATE TABLE Ordre (
  OrdreID   INT NOT NULL,
  OrdreDato DATE DEFAULT NULL,
  KundeID   INT  DEFAULT NULL,
  CONSTRAINT Ordre_pk PRIMARY KEY (OrdreID),
  CONSTRAINT Ordre_fk FOREIGN KEY (KundeID) REFERENCES Kunde (KundeID)
)
  ENGINE = InnoDB;

PRODUKT table:

CREATE TABLE Produkt (
  ProduktID          INT NOT NULL,
  ProduktBeskrivelse VARCHAR(100) DEFAULT NULL,
  ProduktFarge       VARCHAR(20)  DEFAULT NULL,
  Enhetpris          INT          DEFAULT NULL,
  CONSTRAINT Produkt_pk PRIMARY KEY (ProduktID)
)
  ENGINE = InnoDB;

and ORDRELINJE table:

CREATE TABLE Ordrelinje (
  Ordre         INT NOT NULL,
  Produkt       INT NOT NULL,
  AntallBestilt INT DEFAULT NULL,
  CONSTRAINT Ordrelinje_pk PRIMARY KEY (Ordre, Produkt),
  CONSTRAINT Ordrelinje_fk FOREIGN KEY (Ordre) REFERENCES Ordre (OrdreID),
  CONSTRAINT Ordrelinje_fk1 FOREIGN KEY (Produkt) REFERENCES Produkt (ProduktID)
)
  ENGINE = InnoDB;

so when I try to insert values into ORDRELINJE table i get:

Error Code: 1452. Cannot add or update a child row: a foreign key constraint fails (srdjank.Ordrelinje, CONSTRAINT Ordrelinje_fk FOREIGN KEY (Ordre) REFERENCES Ordre (OrdreID))

I've seen the other posts on this topic, but no luck. Am I overseeing something or any idea what to do?

The Problem is with FOREIGN KEY Constraint. By Default (SET FOREIGN_KEY_CHECKS = 1). FOREIGN_KEY_CHECKS option specifies whether or not to check foreign key constraints for InnoDB tables. MySQL - SET FOREIGN_KEY_CHECKS

We can set foreign key check as disable before running Query. Disable Foreign key.

Execute one of these lines before running your query, then you can run your query successfully. :)

1) For Session (recommended)

SET FOREIGN_KEY_CHECKS=0;

2) Globally

SET GLOBAL FOREIGN_KEY_CHECKS=0;

This error generally occurs because we have some values in the referencing field of the child table, which do not exist in the referenced/candidate field of the parent table.

Sometimes, we may get this error when we are applying Foreign Key constraints to existing table(s), having data in them already. Some of the other answers are suggesting to delete the data completely from child table, and then apply the constraint. However, this is not an option when we already have working/production data in the child table. In most scenarios, we will need to update the data in the child table (instead of deleting them).

Now, we can utilize Left Join to find all those rows in the child table, which does not have matching values in the parent table. Following query would be helpful to fetch those non-matching rows:

SELECT child_table.* 
FROM child_table 
LEFT JOIN parent_table 
  ON parent_table.referenced_column = child_table.referencing_column 
WHERE parent_table.referenced_column IS NULL

Now, you can generally do one (or more) of the following steps to fix the data.

1. Based on your "business logic", you will need to update/match these unmatching value(s), with the existing values in the parent table. You may sometimes need to set them null as well.
2. Delete these rows having unmatching values.
3. Add new rows in your parent table, corresponding to the unmatching values in the child table.

Once the data is fixed, we can apply the Foreign key constraint using ALTER TABLE syntax.

In my case the tables were perfectly consistent.

Anyway I was getting this error because I created (by accident) more than one FK constraint on the same field.

I run the following query to show all the keys:

SELECT *
FROM information_schema.table_constraints
WHERE constraint_schema = 'my_db_name'

and I deleted the wrong ones with the following query:

ALTER TABLE my_table
DROP FOREIGN KEY wrong_fk_constraint; 

You can check it also running this query:

SHOW CREATE TABLE my_table;

This can be fixed by inserting the respective records in the Parent table first and then we can insert records in the Child table's respective column. Also check the data type and size of the column. It should be same as the parent table column,even the engine and collation should also be the same. TRY THIS! This is how I solved mine. Correct me if am wrong.

The answer of your question is that you must set the same value in Primary and secondary key. Thanks

check the no. of record in parent table that matches with child table and also primary key must match with foreign key reference. This works for me.

I was getting this issue even though my parent table had all the values I was referencing in my child table. The issue seemed to be that I could not add multiple child references to a single foreign key. In other words if I had five rows of data referenced the same foreign key, MySQL was only allowing me to upload the first row and giving me the error 1452.

What worked for me was typing the code "SET GLOBAL FOREIGN_KEY_CHECKS=0". After that I closed out of MySQL and then restarted it and I was able to upload all of my data with no errors. I then typed "SET GLOBAL FOREIGN_KEY_CHECKS=1" to set the system back to normal although I'm not entirely sure what FOREIGN_KEY_CHECKS does. Hope this helps!

in the foreign key table has a value that is not owned in the primary key table that will be related, so you must delete all data first / adjust the value of your foreign key table according to the value that is in your primary key

you should insert at least one raw in each tables (the ones you want the foreign keys pointing at) then you can insert or update the values of the foreign keys

This helped me out after reading @Mr-Faizan's and other answers.

Untick the 'Enable foreign key checks'

in phpMyAdmin and hit the query. I don't know about WorkBench but the other answers might help you out.

While inserting the foreign key attribute values, first verify the attributes type, as well as primary key attribute value in the parent relation, if the values in parent relation matches, then you can easily insert/update child attribute values.

ill squeeze this in here: my case was trying to create a like for a post which dint exist; while committing to database the error was raised. solution was to first create the post then like it. from my understanding if the post_id was to be saved in the likes table it had to first check with posts table to ascertain existence. i found it better to have it this way since its more logical to me that way..

You must delete data in the child table which does not have any corresponding foreign key value to the parent table primary key .Or delete all data from the child table then insert new data having the same foreign key value as the primary key in the parent table . That should work . Here also a youtube video

Your ORDRELINJE table is linked with ORDER table using a foreign key constraint constraint Ordrelinje_fk foreign key(Ordre) references Ordre(OrdreID) according to which Ordre int NOT NULL, column of table ORDRELINJE must match any Ordre int NOT NULL, column of ORDER table.

Now what is happening here is, when you are inserting new row into ORDRELINJE table, according to fk constraint Ordrelinje_fk it is checking ORDER table if the OrdreID is present or not and as it is not matching with any OrderId, Compiler is complaining for foreign key violation. This is the reason you are getting this error.

Foreign key is the primary key of the other table which you use in any table to link between both. This key is bound by the foreign key constraint which you specify while creating the table. Any operation on the data must not violate this constraint. Violation of this constraint may result in errors like this.

Hope I made it clear.

you should add data from REFERENCES KEY in PRIMARY TABLE to FOREIGN KEY in CHILD TABLE
it means do not add random data to foreign key ? just use data from primary key that is accessable

description of data in foreign key

Taken from Using FOREIGN KEY Constraints

Foreign key relationships involve a parent table that holds the central data values, and a child table with identical values pointing back to its parent. The FOREIGN KEY clause is specified in the child table.

It will reject any INSERT or UPDATE operation that attempts to create a foreign key value in a child table if there is no a matching candidate key value in the parent table.

So your error Error Code: 1452. Cannot add or update a child row: a foreign key constraint fails essentially means that, you are trying to add a row to your Ordrelinje table for which no matching row (OrderID) is present in Ordre table.

You must first insert the row to your Ordre table.

You are getting this constraint check because Ordre table does not have reference OrdreID provided in insert command.

To insert value in Ordrelinje, you first have to enter value in Ordre table and use same OrdreID in Orderlinje table.

Or you can remove not null constraint and insert a NULL value in it.

The problem occurs because you set the foreign key in child table after you insert some data in the child table.

Try removing all the data from child table, then set the foreign key and afterwards add/insert the data in table, it will work.

First allow NULL on the parent table and set the default values to NULL. Next create the foreign key relationship. Afterwards, you can update the values to match accordingly

I had the same problem. I was creating relationships on existing tables but had different column values, which were supposed/assumed to be related. For example, I had a table USERS that had a column USERID with rows 1,2,3,4,5. Then I had another child table ORDERS with a column USERID with rows 1,2,3,4,5,6,7. Then I run MySQl command ALTER TABLE ORDERS ADD CONSTRAINT ORDER_TO_USER_CONS FOREIGN KEY (ORDERUSERID) REFERENCES USERS(USERID) ON DELETE SET NULL ON UPDATE CASCADE;

It was rejected with the message:

Error Code: 1452. Cannot add or update a child row: a foreign key constraint fails (DBNAME1.#sql-4c73_c0, CONSTRAINT ORDER_TO_USER_CONS FOREIGN KEY (ORDERUSERID) REFERENCES USERS (USERID) ON DELETE SET NULL ON UPDATE CASCADE)

I exported data from the ORDERS table, then deleted all data from it, re-run the command again, it worked this time, then re-inserted the data with the corresponding USERIDs from the USERS table.

I know this thread is long already but when I ran a laravel seeding for category tables it created some strange ids which were 1, 11, 21, 31 which caused me error as I tried to reference it in another another table using ids like 1, 2, 3 ... the whole day until I checked the database up manually,

So sometimes go look up the database table to ensure the foreign data actually exist properly.

Cheers

When you're using foreign key, your order of columns should be same for insertion.

For example, if you're adding (userid, password) in table1 from table2 then from table2 order should be same (userid, password) and not like (password,userid) where userid is foreign key in table2 of table1.

I am having the same issue here is my scenario

i put empty('') where value is NULL now this '' value does not match with the parent table's id

here is things need to check , all value with presented in parent table otherwise remove data from parent table then try