Question

CREATE  TABLE IF NOT EXISTS `foo` (
  `foo_id` INT NOT NULL AUTO_INCREMENT ,
  `unique` CHAR(255) NULL ,
  `not_unique` CHAR(255) NULL ,
  PRIMARY KEY (`foo_id`) ,
  UNIQUE INDEX `unique_UNIQUE` (`unique` ASC) )
ENGINE = InnoDB;

This is the table.

INSERT INTO foo (`unique`,`not_unique`) VALUES ('John','Doe')
  ON DUPLICATE KEY UPDATE `foo_id`=LAST_INSERT_ID(`foo_id`);

SELECT LAST_INSERT_ID();

LAST_INSERT_ID here returns 1. That is correct.

INSERT INTO foo (`unique`,`not_unique`) VALUES ('John','Doe')
  ON DUPLICATE KEY UPDATE `foo_id`=LAST_INSERT_ID(`foo_id`);

SELECT LAST_INSERT_ID();

LAST_INSERT_ID here returns 1. That is correct.

INSERT INTO foo (`unique`,`not_unique`) VALUES ('Jane','Doe')
  ON DUPLICATE KEY UPDATE `foo_id`=LAST_INSERT_ID(`foo_id`);

SELECT LAST_INSERT_ID();

LAST_INSERT_ID here returns 3. Why? I was hoping it to be 2. If this is a bug, is there a workaround for it?

Answer 1

The id was taken at the beginning of the attempted insert, and was discarded on failure.