duplicate key error does not cancel/rollback mysql transaction

Question

When in a mysql innodb transaction, I would expect a duplicate key error to cause a rollback. It doesn't, instead it simply throws an error and continues on to the next command. Once the COMMIT command is reached, the transaction will be committed, sans the duplicate key causing command.

Is this the expected behaviour? If so, how would one go about setting it up so that the transaction is rolled back instead of committed when such an error occurs?

test environment:

CREATE TABLE `test` (  
  `id` int(11) NOT NULL, 
  PRIMARY KEY (`id`) 
) ENGINE=InnoDB DEFAULT CHARSET=latin1 

BEGIN;
     INSERT INTO test VALUES (5);
     INSERT INTO test VALUES (5);
COMMIT;

expected result: table test is empty

actual result: table test contains one record with value 5

Answer 1

MySql (and other sql engine AFAIK) does not automatically rollback a transaction if an error occurs.

You have to declare an error handler that will Rollback the transaction:

DECLARE EXIT HANDLER FOR SQLEXCEPTION, SQLWARNING, NOT FOUND
BEGIN 
  ROLLBACK; 
  CALL ERROR_ROLLBACK_OCCURRED; 
END;

Answer 2

If an insert fails because of a duplicate, the database rolls the transaction back to the start of that statement.

It uses an internal savepoint made at the beginning of the statement, then rolls back to that savepoint.

It does NOT roll back the entire transaction, because that might not have been what you wanted.

The behaviour of the mysql client is configurable using command line parameters. It can either quit (which would implicitly rollback) or continue.

If you're using your own app, what it does is up to you.

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Mysql does not impose POLICY on how you handle failures - it leaves that up to your application. So what you do about them is your own business - you can ignore them if you like.