Question

When I run a mysqldump command on my database and then try to import it, it fails as it attempts to create the tables alphabetically, even though they may have a foreign key that references a table later in the file. There doesn’t appear to be anything in the documentation and I’ve found answers like this that say to update the file after it’s created to include:

set FOREIGN_KEY_CHECKS = 0;
...original mysqldump file contents...
set FOREIGN_KEY_CHECKS = 1;

Is there no way to automatically set those lines or export the tables in the necessary order (without having to manually specify all table names as that can be tedious and error prone)? I could wrap those lines in a script, but was wondering if there is an easy way to ensure I can dump a file and then import it without manually updating it.

Answer 1

The mysqldump command included with MySQL since version 4.1.1 by default produces a script that turns off the foreign key checks. The following line is included near the top of the dump file:

/*!40014 SET @OLD_FOREIGN_KEY_CHECKS=@@FOREIGN_KEY_CHECKS, FOREIGN_KEY_CHECKS=0 */;

The /*!40014 ... */ syntax is a conditional comment that will be executed on MySQL version 4.0.14 and later. The old foreign key checks setting is restored towards the end of the dump file:

/*!40014 SET FOREIGN_KEY_CHECKS=@OLD_FOREIGN_KEY_CHECKS */;

Note that the conditional comments are interpreted by the the client (rather than the server). If you load the dump file with a client that doesn’t support them, then foreign key checks will not be disabled and you might encounter errors. For best results, I’d suggest loading dump files using the official mysql command line client:

mysql -hserver -uuser -p database < dumpfile.sql

It’s also worth noting that if mysqldump is run with the --compact option, then the commands to disable and re-enable the foreign key checks are omitted from the dump file.