Question

brief:

I’m experiencing strange MySQL behavior – on update CURRENT_TIMESTAMP attribute is being added although I don’t want it to be added. I want to find out why is this happening – is it a matter of MySQL server or MySQL Workbench I’m using (v5.2.38).

detail:

I’ve modelled the database structure using EER diagrams, an example table is below:

CREATE  TABLE IF NOT EXISTS `privilege` (
  `id` INT NOT NULL ,
  `name` VARCHAR(64) NOT NULL COMMENT 'privilege name (just a label)' ,
  `created_at` TIMESTAMP NOT NULL COMMENT 'when the privilege was created' ,
  PRIMARY KEY (`id`) )
ENGINE = InnoDB;

The above script fragment is created using Export > Forward Engineer SQL Create Script inside WorkBench. The created_at column is siginifant here. It is NOT NULL and there is no default value defined for the timestamp when a record is defined. So I guess, that if someone tries to insert a record without defining created_at, an error will be raised.

I run this script inside MySQL server to create the whole structure. And the created structure is different – show create table privilege returns the following:

CREATE TABLE `privilege` (
  `id` int(11) NOT NULL,
  `name` varchar(64) NOT NULL COMMENT 'privilege name (just a label)',
  `created_at` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE
  CURRENT_TIMESTAMP COMMENT 'when the privilege was created',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

Where did “on update current_timestamp” come from? I’m 100% sure that I didn’t choose any appropriate option, so MySQL should not create anything he’s not asked to.

Does anyone have an idea why those clauses are added?

Answer 1

As documented under Automatic Initialization and Updating for TIMESTAMP:

The following rules describe the possibilities for defining the first TIMESTAMP column in a table with the current timestamp for both the default and auto-update values, for one but not the other, or for neither:

[ deletia ]

• With neither DEFAULT CURRENT_TIMESTAMP nor ON UPDATE CURRENT_TIMESTAMP, it is the same as specifying both DEFAULT CURRENT_TIMESTAMP and ON UPDATE CURRENT_TIMESTAMP.

[ deletia ]

To suppress automatic properties for the first TIMESTAMP column, do either of the following:

• Define the column with a DEFAULT clause that specifies a constant default value.

• Specify the NULL attribute. This also causes the column to permit NULL values, which means that you cannot assign the current timestamp by setting the column to NULL. Assigning NULL sets the column to NULL.