[Viet]

Hola , como bien te comenta el amigo jachguate todo depende el motod de base de datos que utilices.....

por ejemplo en SQL Server lo puedes hacer con :

 Syntax

ALTER TABLE table
{	[ALTER COLUMN column_name
		{	new_data_type [ (precision[, scale] ) ]
					[ NULL | NOT NULL ]
			| {ADD | DROP} ROWGUIDCOL
		}
	]
	| ADD
		{	[ <column_definition> ]
			|  column_name AS computed_column_expression
		}[,...n]
	| [WITH CHECK | WITH NOCHECK] ADD
		{ <table_constraint> }[,...n]
	| DROP
		{	[CONSTRAINT] constraint_name
			| COLUMN column
		}[,...n]
	| {CHECK | NOCHECK} CONSTRAINT
		{ALL | constraint_name[,...n]}
	| {ENABLE | DISABLE} TRIGGER
		{ALL | trigger_name[,...n]}
}

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