create table E1 (

A1 int null primary key,

B1 int null,

AA int null unique,

constraint FK_X foreign key (AA)

references E1(A1) )

create table E1 (

A1 int not null primary key,

B1 int null,

AA int null,

constraint FK_X foreign key (A1)

references E1(AA) )

create table E1 (

A1 int not null unique,

B1 int null,

AA int null primary key,

constraint FK_X foreign key (AA)

references E1(A1) )

create table E1 (

A1 int not null primary key,

B1 int null,

AA int null unique,

constraint FK_X foreign key (A1)

references E1(AA) )

create table E1 (

A1 int not null primary key,

B1 int null,

AA int null unique,

constraint FK_X foreign key (AA)

references E1(A1) )