Oracle in a Nutshell

Name

Column_Constraint_Clause

Synopsis

[CONSTRAINT constraint_name]
{ {NULL | NOT NULL}
| {UNIQUE | PRIMARY KEY (column[,column ...])}
| REFERENCES [schema.]table_name [(column[,column ...]) 
  [ON DELETE {CASCADE | SET NULL}] 
| CHECK (condition) 
}
[Constraint_State_Clause]

Defines column constraints.

Keywords

CONSTRAINT

Specifies the name for the constraint. If omitted, Oracle will assign a name in the form SYS_Cnnn, where nnn is an integer number.

NULL

Specifies that the column may contain a value of NULL.

NOT NULL

Specifies that the column may not contain a value of NULL.

UNIQUE

Specifies that the value in this column must not duplicate a value for the same column in any other row of the table; that is, the value of this column must be unique for all rows of the table. Note, however, that more than one row may contain NULL in this column.

PRIMARY KEY

Specifies that the column or combination of columns serves as the primary key for the table in which it is defined. A primary key column may not contain NULL, and no value of the primary key may appear in the same column of any other row in the table (i.e., the key value must be unique).

REFERENCES

Specifies the table and column(s) that are referenced by a foreign key. The combination of referenced columns must represent either a PRIMARY KEY constraint or a UNIQUE constraint on the target table.

ON DELETE CASCADE

Specifies that when a row containing a primary or unique key is deleted, rows containing dependent foreign keys will also be automatically ...