How can i solve the unique constraint error in sql program? - sql

Question

I have homework to create tables and insert data into it and when i insert data into Branch table it gives me a unique constraint error and I don’t know why, and how can I solve this problem. my codes:
create table Branch(
Branchno varchar (4) primary key not null,
Street varchar (15) not null,
City varchar (10) not null,
Postcode varchar (10) not null)
create table staff (
Staffno varchar (4),
Fname varchar2 (50) not null,
Lname varchar (50),
Position varchar2 (50),
Sex varchar (1),
Dob date,
Salary number (30),
Branchno varchar (4),
constraint pk_staff primary key (staffno),
constraint fk_staff_branchno foreign key (branchno)
references Branch (branchno)
)
create table Client
(Clientno varchar (4) primary key not null,
Fname varchar (50) not null,
Lname varchar (50) not null,
Telno varchar (50) not null,
Preftype varchar (18) not null,
Maxrent INT)
create table PropertyForRent(
Propertyno varchar (40) not null,
Street varchar (40),
City varchar (40),
Postcode varchar (40),
Type varchar (40),
Rooms number (30),
Rent number (30),
Ownerno varchar (40),
Staffno varchar (40),
Branchno varchar (40),
constraint pk_PropertyForRent primary key (Propertyno),
foreign key (staffno)
references staff (staffno),
foreign key (branchno)
references branch (branchno)
)
create table Registration
(clientno varchar (60) not null,
branchno varchar (50) not null,
staffno varchar (50) not null,
datejoined date not null,
foreign key (clientno) references client (clientno),
foreign key (branchno) references branch (branchno),
foreign key (staffno) references staff (staffno)
)
The data that I insert to give me this error:
INSERT INTO Branch values
('B005','22 Deer Rd','London','SW1 4EH');
INSERT INTO Branch values
('B007','16 Argy11 St','Aberdeen','AB2 3SU');

Answer 1

I try with Oracle 11g and not receive any error:
SQL> INSERT INTO Branch values
('B005','22 Deer Rd','London','SW1 4EH'); 2
1 row created.
SQL> INSERT INTO Branch values
('B007','16 Argy11 St','Aberdeen','AB2 3SU'); 2
1 row created.
SQL> commit;
Commit complete.
Probably have you already record with these keys or have you any trigger on this table.

Answer 2

Instead of an INSERT INTO
you could
MERGE INTO Branch dest
USING( SELECT 'B005' Branchno,'22 Deer Rd' Street,'London' City,'SW1 4EH' Postcode FROM dual) src
ON( dest.Branchno = src.Branchno )
WHEN NOT MATCHED THEN
INSERT( Branchno, Street, City,Postcode )
VALUES ( src.Branchno, src.Street, src.City,src.Postcode );
That looks on the first sight, more difficult, but you would not get an error and all stops.
trggers can't stop you from inserting doulbe entires as they you can only can raise errors with wuld stop again every multi insert
see example