PRIMARY KEY
Derby Reference Manual
75
Table-level constraints refer to one or more columns in the table. Table-level
constraints specify the names of the columns to which they apply. Table-level
CHECK constraints can refer to 0 or more columns in the table.
Column constraints include:
· NOT NULL
Specifies that this column cannot hold NULL values (constraints of this type are not
nameable).
· PRIMARY KEY
Specifies the column that uniquely identifies a row in the table. The identified
columns must be defined as NOT NULL.
Note: If you attempt to add a primary key using ALTER TABLE and any of the
columns included in the primary key contain null values, an error will be generated
and the primary key will not be added. See
information.
· UNIQUE
Specifies that values in the column must be unique.
· FOREIGN KEY
Specifies that the values in the column must correspond to values in a referenced
primary key or unique key column or that they are NULL.
· CHECK
Specifies rules for values in the column.
Table constraints include:
· PRIMARY KEY
Specifies the column or columns that uniquely identify a row in the table. NULL
values are not allowed.
· UNIQUE
Specifies that values in the columns must be unique.
· FOREIGN KEY
Specifies that the values in the columns must correspond to values in referenced
primary key or unique columns or that they are NULL.
Note: If the foreign key consists of multiple columns, and any column is NULL,
the whole key is considered NULL. The insert is permitted no matter what is on the
non-null columns.
· CHECK
Specifies a wide range of rules for values in the table.
Column constraints and table constraints have the same function; the difference is
in where you specify them. Table constraints allow you to specify more than one
column in a PRIMARY KEY, UNIQUE, CHECK, or FOREIGN KEY constraint definition.
Column-level constraints (except for check constraints) refer to only one column.
A constraint operates with the privileges of the owner of the constraint. See "Using SQL
standard authorization" and "Privileges on views, triggers, and constraints" in the Derby
Developer's Guide for details.
Syntax
Primary key constraints
A primary key defines the set of columns that uniquely identifies rows in a table.
