MYSQL won't drop the default value for a timestamp - mysql

Question

I am trying to drop the default value of a timestamp column.
My DDL looks something like this:
CREATE TABLE bills
(
...
created_at TIMESTAMP DEFAULT '1971-01-06 00:00:00',
updated_at TIMESTAMP DEFAULT '1970-01-01 00:00:01',
);
When I try any of the following:
ALTER TABLE bills ALTER COLUMN created_at DROP DEFAULT;
ALTER TABLE bills ALTER COLUMN created_at SET DEFAULT NULL;
ALTER TABLE bills MODIFY COLUMN created_at TIMESTAMP NULL;
ALTER TABLE bills MODIFY COLUMN created_at TIMESTAMP NULL DEFAULT NULL;
ALTER TABLE bills CHANGE COLUMN created_at created_at TIMESTAMP NULL;
ALTER TABLE bills CHANGE COLUMN created_at created_at TIMESTAMP NULL DEFAULT NULL;
The MYSQL shell says it has executed correctly.
However, when I check the DDL again, it is clear nothing has changed.
On the other hand, I can alter the default value to a specified value like so:
ALTER TABLE bills ALTER COLUMN created_at SET DEFAULT '1972-02-02 00:00:00';
And it updates correctly.
Note that the columns are nullable (as you can see by the DDL) and I have put null data values in the "create_at" column.
The MYSQL version I am using is as v5.7.12
Here is more details about the version I am using:
$ mysql --version
mysql Ver 14.14 Distrib 5.7.21, for Linux (x86_64) using EditLine wrapper

Answer 1

Use the CHANGE COLUMN or MODIFY COLUMN versions of the ALTER TABLE syntax. These require you to specify all the attributes of the column that you want to carry over from the old to the new definition (CHANGE also requires the name so that you can rename at the same time).
e.g.
ALTER TABLE bills CHANGE COLUMN created_at created_at TIMESTAMP NULL;
ALTER TABLE bills MODIFY COLUMN updated_at TIMESTAMP NULL;
Here's a sqlfiddle so you can see it working http://sqlfiddle.com/#!9/303110/3
Update
Have you checked the value of the EXPLICIT_DEFAULTS_FOR_TIMESTAMP variable?
Following on from my comments below, I've now succeeded in getting both of your statements to work in mysql 5.7 by setting the variable EXPLICIT_DEFAULTS_FOR_TIMESTAMP = 1;
I'm afraid I can't properly explain the purpose of this variable except to say that it appears to be intended to disable some 'nonstandard' behaviours in TIMESTAMP fields. It's documented here.
I'll quote one bit of this
By default, explicit_defaults_for_timestamp is disabled, which enables
the nonstandard behaviors.
Personally, I think this is the wrong way round, you ought to set the variable to allow the use of 'nonstandard' behaviours if you wish to go down that path. I'd call that a bug.
In your case, and assuming this actually works on your system, disabling the 'nonstandard' behaviour would allow the 'standard' syntax for changing a column default to work.
This is the output on my system
paul#zoltan:~$ mysql --version
mysql Ver 14.14 Distrib 5.7.21, for Linux (x86_64) using EditLine wrapper
mysql> SET ##SESSION.EXPLICIT_DEFAULTS_FOR_TIMESTAMP = 1;
Query OK, 0 rows affected (0.00 sec)
mysql> drop table bills;
Query OK, 0 rows affected (0.17 sec)
mysql> create table bills (
-> id INT AUTO_INCREMENT PRIMARY KEY,
-> created_at TIMESTAMP DEFAULT '1971-01-06 00:00:00',
-> updated_at TIMESTAMP DEFAULT '1970-01-01 01:00:01'
-> );
Query OK, 0 rows affected (0.42 sec)
mysql> alter table bills alter column created_at drop default;
Query OK, 0 rows affected (0.10 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> alter table bills alter column created_at set default null;
Query OK, 0 rows affected (0.06 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> alter table bills alter column updated_at drop default;
Query OK, 0 rows affected (0.05 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> alter table bills alter column updated_at set default null;
Query OK, 0 rows affected (0.04 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> show create table bills\G
*************************** 1. row ***************************
Table: bills
Create Table: CREATE TABLE `bills` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`created_at` timestamp NULL DEFAULT NULL,
`updated_at` timestamp NULL DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1
1 row in set (0.00 sec)