Insert self-referencing data into copy table - mysql

Question

I have a self-referencing foreign-key constraint on my table. Because I'm transforming the schema, I'd like to copy the existing data into a new table with (more or less) the same structure:
CREATE TABLE test(
id INT NOT NULL PRIMARY KEY,
parent INT,
FOREIGN KEY (parent) REFERENCES test(id)
);
CREATE TABLE copy(
id INT NOT NULL PRIMARY KEY,
parent INT,
FOREIGN KEY (parent) REFERENCES copy(id)
);
However, when inserting the data:
INSERT INTO copy(id, parent) SELECT id, parent FROM test;
MySQL gives me the common integrity error:
Error Code: 1452. Cannot add or update a child row:
a foreign key constraint fails (`test`.`copy`, CONSTRAINT `copy_ibfk_1`
FOREIGN KEY (`parent`) REFERENCES `copy` (`id`))
It seems that MySQL checks the constraint after inserting each row instead of checking it after the whole insert. The exact same example works fine in PostgreSQL.
Is there any other way to insert this data or am I stuck with doing this in two steps like this?
INSERT INTO copy(id) SELECT id FROM test;
UPDATE copy
JOIN test ON test.id = copy.id
SET copy.parent = test.parent;

Answer 1

You can use
SET FOREIGN_KEY_CHECKS=0;
before your INSERT and
SET FOREIGN_KEY_CHECKS=1;
after to disable the check of the foreign key constraint, so you don't have to worry about the order of the inserts:
See the documentation to foreign_key_checks
foreign_key_checks
If set to 1 (the default), foreign key constraints for InnoDB tables
are checked. If set to 0, such constraints are ignored. [...] Typically
you leave this setting enabled during normal operation, to enforce
referential integrity. Disabling foreign key checking can be useful
for reloading such tables in an order different from that required by
their parent/child relationships. See Section 14.6.6, “InnoDB and
FOREIGN KEY Constraints”. [...]
Note
Setting foreign_key_checks to 1 does not trigger a scan of the
existing table data. Therefore, rows added to the table while
foreign_key_checks = 0 will not be verified for consistency.
CREATE TABLE test(
id INT NOT NULL PRIMARY KEY,
parent INT,
FOREIGN KEY (parent) REFERENCES test(id)
);
CREATE TABLE copy(
id INT NOT NULL PRIMARY KEY,
parent INT,
FOREIGN KEY (parent) REFERENCES copy(id)
);
INSERT INTO test(id, parent) VALUES(1, null);
INSERT INTO test(id, parent) VALUES(2, 1);
INSERT INTO test(id, parent) VALUES(3, null);
INSERT INTO test(id, parent) VALUES(4, 2);
UPDATE test SET parent=3 WHERE id=1;
SET FOREIGN_KEY_CHECKS=0;
-- Success with MySQL
INSERT INTO copy(id, parent) SELECT id, parent FROM test;
SET FOREIGN_KEY_CHECKS=1;
Your updated fiddle: http://sqlfiddle.com/#!2/ae623/1