Normally, errors occurs for data-change statements (such as INSERT or UPDATE) that would violate primary-key, unique-key, or foreign-key constraints. If you are using a transactional storage engine such as InnoDB, MySQL automatically rolls back the statement. If you are using a non-transactional storage engine, MySQL stops processing the statement at the row for which the error occurred and leaves any remaining rows unprocessed.

MySQL supports an IGNORE keyword for INSERT, UPDATE, and so forth. If you use it, MySQL ignores primary-key or unique-key violations and continues processing with the next row. See the section for the statement that you are using (Section 12.2.4, “INSERT Syntax”, Section 12.2.10, “UPDATE Syntax”, and so forth).

You can get information about the number of rows actually inserted or updated with the mysql_info() C API function. You can also use the SHOW WARNINGS statement. See Section 26.2.3.35, “mysql_info()”, and Section 12.5.5.33, “SHOW WARNINGS Syntax”.

Currently, only InnoDB tables support foreign keys. See Section 13.2.6.4, “FOREIGN KEY Constraints”. We plan to add foreign key support by other storage engines in a future MySQL release. See Section 1.5, “MySQL Development Roadmap”.