Sybase 12.4.2 User Manual

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

.

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”