Data often must adhere to certain restrictions or rules.
Such restrictions might apply to single pieces of information, such
as the format and sequence numbers, or they might apply to several
pieces of information.
- Nullability of column data values
- Null values represent unknown states. By default, all of the built-in
data types support the presence of null values. However, some business
rules might dictate that a value must always be provided for some
columns, for example, emergency information. For this condition, you
can use the NOT NULL constraint to ensure that a given column of a
table is never assigned the null value. Once a NOT NULL constraint
has been defined for a particular column, any insert or update operation
that attempts to place a null value in that column will fail.
- Default column data values
- Just as some business rules dictate that a value must always be
provided, other business rules can dictate what that value should
be, for example, the gender of an employee must be either M or F.
The column default constraint is used to ensure that a given column
of a table is always assigned a predefined value whenever a row that
does not have a specific value for that column is added to the table.
The default value provided for a column can be null, a constraint
value that is compatible with the data type of the column, or a value
that is provided by the database manager. For more information, see: Default column and data type definitions.
- Keys
- A key is a single column or a set of columns in a table or index
that can be used to identify or access a specific row of data. Any
column can be part of a key and the same column can be part of more
than one key. A key that consists of a single column is called an
atomic key; a key that is composed of more than one column is called
a composite key. In addition to having atomic or composite attributes,
keys are classified according to how they are used to implement constraints:
- A unique key is used to implement unique constraints.
- A primary key is used to implement entity integrity constraints.
(A primary key is a special type of unique key that does not support
null values.)
- A foreign key is used to implement referential integrity constraints.
(Foreign keys must reference primary keys or unique keys; foreign
keys do not have corresponding indexes.)
Keys are normally specified during the declaration of a table,
an index, or a referential constraint definition.
- Constraints
- Constraints are rules that limit the values that can be inserted,
deleted, or updated in a table. There are check constraints, primary
key constraints, referential constraints, unique constraints, unique
key constraints, foreign key constraints, and informational constraints.
For details about each of these types of constraints, see: Constraints or Types of constraints.
DB2 Version 10.1 for Linux, UNIX, and Windows