Oracle SQL: the Essential Reference

Table_Constraint_Clause

[CONSTRAINT constraint_name] {[UNIQUE | PRIMARY KEY] (column[,column ...])] | [CHECK (condition) ] | [FOREIGN KEY (column[,column ...]) REFERENCES [schema.]table [(column[,column ...])] [ON DELETE {CASCADE | SET NULL}]] } [Constraint_State_Clause]

Defines constraints on a table.

Keywords

CONSTRAINT

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

UNIQUE

Specifies that the value in the column(s) must not duplicate a value for the same column(s) in any other row of the table. The value of the column or columns in the constraint must be unique for each row in the table. Note, however, that more than one column may contain the value NULL.

PRIMARY KEY

Specifies that the column(s) identified by column will serve 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(s) of any other row in the table.

CHECK

Specifies a condition that must evaluate to TRUE or NULL for the constraint to be satisfied.

FOREIGN KEY

Specifies that one or more columns in this table participate in a foreign key referential integrity relationship.

REFERENCES

Specifies the table and column(s) that are referenced by this foreign key constraint.

ON DELETE CASCADE

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

ON DELETE SET NULL ...