| Constraints enable the RDBMS enforce the
integrity of the database automatically, without needing you to create
triggers, rule or defaults. |
| Different types of constraints. |
- NOT NULL specifies that the
column does not accept NULL values.
- UNIQUE constraints enforce
the uniqueness of the values in a set of columns.
- PRIMARY KEY constraints
identify the column or set of columns whose values uniquely identify
a row in a table.
- CHECK constraints enforce
domain integrity by limiting the values that can be placed in a
column.
- FOREIGN KEY constraints
identify the relationships between tables.
|
| Column and Table Constraints |
- A column constraint is specified as
part of a column definition and applies only to that column (the
constraints in the earlier samples are column constraints).
- A table constraint is declared
independently from a column definition and can apply to more than
one column in a table.
Table constraints must be used when more
than one column must be included in a constraint. |
