Why 'timestamp not null' is resulting in default being set to 'CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP'?

Question

When I create a new table

create table test(rId int auto_increment, createtimestamp timestamp not null, PRIMARY KEY (rId));

it creates a table with default schema for createtimestamp as CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP .

Expected behavior was no default set. But it's getting set to default: CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP . Why is this happening?

I can alter this after the creation with ALTER TABLE test CHANGE COLUMN createtimestamp createtimestamp TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ; but can't alter it to ALTER TABLE test CHANGE COLUMN createtimestamp createtimestamp TIMESTAMP NOT NULL ; .

It looks like a default has to be specified but this is exactly what I don't want. How can I achieve a behavior where the default is not set and schema is not null, just after creation of table and without any alteration required?

MySql Version: 5.5.49

Answer 1

初始答案是正确的，但如果在定义中有一个默认值或NULL是不可接受的，那么可以考虑使用datetime，或者升级到允许行为更改的MySQL 5.6 http://dev.mysql.com/doc /refman/5.6/en/server-system-variables.html#sysvar_explicit_defaults_for_timestamp 。