Question

In the following table definition, what is the difference between these two column definitions, or does the second just create an automatically named foreign key index?

CREATE TABLE dbo.Employee
(
    dept_id     int  NOT NULL  
        CONSTRAINT fk_employee_deptid FOREIGN KEY REFERENCES Department(dept_id),
    empType_id  int  NOT NULL  REFERENCES  EmployeeType(empType_id)
    /* ... other columns ... */
);

Answer 1

The only difference is that the second one will be given a system generated name that will likely be more cryptic than one you allocate yourself.

The column name is also optional where there is an unambiguous possibility.

empType_id  int  NOT NULL  REFERENCES  EmployeeType

can also work. Again no difference in the end result. The full grammar for the FK declaration is

[ CONSTRAINT constraint_name ] 
{      [ FOREIGN KEY ] 
        REFERENCES [ schema_name . ] referenced_table_name [ ( ref_column ) ] 
        [ ON DELETE { NO ACTION | CASCADE | SET NULL | SET DEFAULT } ] 
        [ ON UPDATE { NO ACTION | CASCADE | SET NULL | SET DEFAULT } ] 
        [ NOT FOR REPLICATION ] 
 } 

Optional items are enclosed in square brackets.