Alter SQL table - allow NULL column value

问题

Initially, the table "MyTable" has been defined in the following way:

CREATE TABLE IF NOT EXISTS `MyTable` (
  `Col1` smallint(6) NOT NULL AUTO_INCREMENT,
  `Col2` smallint(6) DEFAULT NULL,
  `Col3` varchar(20) NOT NULL,
);

How to update it in such a way that the column "Col 3" would be allowed to be NULL?

回答1:

The following MySQL statement should modify your column to accept NULLs.

ALTER TABLE `MyTable`
ALTER COLUMN `Col3` varchar(20) DEFAULT NULL

回答2:

ALTER TABLE MyTable MODIFY Col3 varchar(20) NULL;

来源：https://stackoverflow.com/questions/10597982/alter-sql-table-allow-null-column-value