我这样写是为了我可以一次将所有的表和列编辑为空:

select 
case
when sc.max_length = '-1' and st.name in ('char','decimal','nvarchar','varchar')
then
'alter table  [' + so.name + '] alter column [' + sc.name + '] ' + st.name + '(MAX) NULL'
when st.name in ('char','decimal','nvarchar','varchar')
then
'alter table  [' + so.name + '] alter column [' + sc.name + '] ' + st.name + '(' + cast(sc.max_length as varchar(4)) + ') NULL'
else
'alter table  [' + so.name + '] alter column [' + sc.name + '] ' + st.name + ' NULL'
end as query
from sys.columns sc
inner join sys.types st on st.system_type_id = sc.system_type_id
inner join sys.objects so on so.object_id = sc.object_id
where so.type = 'U'
and st.name <> 'timestamp'
order by st.name

MySQL, MariaDB

ALTER TABLE [table name] MODIFY COLUMN [column name] [data type] NULL

使用MODIFY COLUMN代替ALTER COLUMN。

是的，你可以使用ALTER TABLE如下:

ALTER TABLE [table name] ALTER COLUMN [column name] [data type] NULL

引用ALTER TABLE文档:

可以在ALTER COLUMN中指定NULL，以强制NOT NULL列允许空值，PRIMARY KEY约束列除外。

最简单的方法是，

alter table table_name change column_name column_name int(11) NULL;

ALTER TABLE正确:

ALTER TABLE MyCustomers ALTER COLUMN CompanyName VARCHAR(20) NULL

ALTER TABLE public.contract_termination_requests
ALTER COLUMN management_company_id DROP NOT NULL;