Differences between “foreign key” and “constraint foreign key”

Question

I mean for example I can create table like

create table XTable
( 
  idt int not null primary key,
  value nvarchar(50),
  idq int,
  constraint fk_idq foreig         


        

Answer 1

The first option is purely for naming the constraint.

From SQL FOREIGN KEY Constraint

To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax

CREATE TABLE Orders
(
  O_Id int NOT NULL,
  OrderNo int NOT NULL,
  P_Id int,
  PRIMARY KEY (O_Id),
  CONSTRAINT fk_PerOrders FOREIGN KEY (P_Id)
  REFERENCES Persons(P_Id)
)

Also, from CREATE TABLE (Transact-SQL) one can see that [ CONSTRAINT constraint_name ] is optional.