Sybase 12.4.2 manual Altering tables, 123

CHAPTER 3 Working with Database Objects

	By contrast, when you specify UNIQUE or PRIMARY KEY, each value can occur
	only once in that column. For example, in the employee table, each value of
	ss_number, the employee’s social security number, can occur just once
	throughout that column. This uniqueness extends to NULL values. Thus, a
	column specified as UNIQUE must also have the constraint NOT NULL.
Altering tables
	This section describes how to change the structure of a table using the ALTER
	TABLE statement.
Example 1	The following command adds a column to the skill table to allow space for an
	optional description of the skill:
	ALTER TABLE skill
	ADD skill_description CHAR( 254 )
Example 2	The following statement changes the name of the skill_type column to
	classification:
	ALTER TABLE skill
	RENAME skill_type TO classification
Example 3	The following statement deletes the classification column.
	ALTER TABLE skill
	DELETE classification
Example 4	The following statement changes the name of the entire table:
	ALTER TABLE skill
	RENAME qualification
	These examples show how to change the structure of the database. The ALTER
	TABLE statement can change many characteristics of a table—foreign keys can
	be added or deleted, and so on. However, you cannot use MODIFY to change
	table or column constraints. Instead, you must DELETE the old constraint and
	ADD the new one. In all these cases, once you make the change, stored
	procedures, views, and any other item referring to this column will no longer
	work.
	For a complete description of the ALTER TABLE command, see Adaptive
	Server IQ Reference Manual. For information about building constraints into
	table definitions using ALTER TABLE, see Chapter 7, “Ensuring Data
	Integrity”

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