ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails

Question

When I execute the following SQL command:

INSERT INTO test_usershosts (RID,userid,hid,Usr,Pass) 
VALUES (NULL,1,1,\"user\",\"pass\");

I\'m

Answer 1

The reason why you are getting that exception is because you are inserting a record on table test_usershosts which the value of the userID is not present on table test_users. Same as the value of hid is not also present on table test_hosts.

Table test_usershosts is dependent on tables: test_users and test_hosts. So be sure that when inserting records on table test_usershosts, the values for hid and userid already exists on the parent tables: test_users and test_hosts.

Try executing this query and surely it will be inserted.

INSERT INTO test_usershosts (RID,userid,hid,Usr,Pass) 
VALUES (NULL,1120,30,'user','pass');

• SQLFiddle Demo

I see that AUTO_INCREMENT option on tables: test_users and test_hosts, are not needed since you are supplying values on every query you are executing on the two tables.

Answer 2

In test_users guest's UserID is 1120 and in test_hosts hid = 30

you must change

INSERT INTO test_usershosts (RID,userid,hid,Usr,Pass) VALUES (NULL,1,1,"user","pass");

to:

INSERT INTO test_usershosts (RID,userid,hid,Usr,Pass) VALUES (NULL,1120,30,"user","pass");

Answer 3

in my case in case it serves someone. I was struggling to find the solution what usually happens when making a primary key is also marked as unique. but in my case it was only marked as primary and it was not marked as unique, until I noticed that the option of unica was "open" and I clicked it, until then I let myself make the relationship.

Then for the record ... check that your primary keys are also unique.