Why multi-table cross-database deletes fail in MySQL
Posted in Databases on May 12, 2006
Sometimes multi-table deletes fail in MySQL with a message about an unknown table. In this article I’ll explain the exact combination of circumstances that cause it to happen.
First, the symptoms:
delete a from db1.t1 as a inner join db1.t2 as b on a.c1 = b.c1; ERROR 1109: Unknown table 'b' in MULTI DELETE
This will happen if the following are true:
According the the MySQL documentation,
Cross-database deletes are supported for multiple-table deletes, but in this case, you must refer to the tables without using aliases.
That’s correct, but it might not seem right. If the tables are fully qualified by database and table name, but are in the current database, it works. If I switch to another database and run the same statement, it won’t work.
This bit me when I was scripting out a large procedure running from a Perl script that doesn’t specify a database. I scripted it while I was connected and had a database specified. Then I ran it, and it bombed out when it got to the delete statement.
The solution is not to alias the tables. It’s less convenient, but it’s the only thing to do sometimes.