A CONSTRAINT definition (constraint_definition) defines an integrity condition that must be fulfilled by all the rows in one table.
<constraint_definition> ::= CHECK <search_condition>
| CONSTRAINT <search_condition>
| CONSTRAINT <constraint_name>
CHECK <search_condition>
SQL Tutorial, Constraints
A CONSTRAINT definition defines an integrity condition that must be fulfilled by all the column values in the columns defined by the column definition with CONSTRAINT definition.
The CONSTRAINT definition in a column is checked when a row is inserted and a column changed that occurs in the CONSTRAINT definition. If the CONSTRAINT definition is violated, the INSERT or UPDATE statement fails.
When you define a constraint, you specify implicitly that the NULL value is not permitted as an input.
The search condition (search_condition) of the CONSTRAINT definition must not contain a subquery and must only contain column names in the form column_name.
·
No constraint name
specified:
The database system assigns a constraint name that is unique for the table in
question.
·
Constraint name
specified:
The constraint name must be different to all other constraint names for this
table.
The number of columns in a search condition plays a role.
·
The search
condition contains only one column name in the table:
When the table is created (CREATE TABLE
statement), you can check whether an additional DEFAULT value specified as
a column
attribute fulfills the search condition. If it is not true, the CREATE
TABLE statement fails.
·
The search
condition contains more than one column name in the table:
When the table is created (CREATE TABLE statement), it is not possible to
decide whether DEFAULT values of the table columns fulfill the search
condition. In this case, an attempt to insert DEFAULT values in the table when
an INSERT or UPDATE statement is executed may fail.
See also: