Very interesting!

CREATE TABLE A(A INT UNIQUE);
INSERT INTO A(A) VALUES(1);
INSERT INTO A(A) VALUES(1);
DROP TABLE A;

CREATE TABLE A(A INT);
CREATE UNIQUE INDEX AA ON A(A);
INSERT INTO A(A) VALUES(1);
INSERT INTO A(A) VALUES(1);
DROP TABLE A;

SQL will raise an error with an error code of 2601 when violating a unique index, while it will use error code 2627 for violating a unique constraint.

Minor difference, but while they are very similar, they are NOT the same.

As you may be aware, Unique Key can allow only one null. Meaning only one null can be accepted not more than that. If you have more than one null on any column, you cannot make that column/field as unique key.

I am sure there must be another way of doing this, I would do something like this,

1. Since there is no way you can make that field a unique key, how about making a composite key.

a) Combine this field with any other unique column/field and make a composite unique key. If any other unique key already exists then add this field to the previous unique key.

b) if you do not have any unique key ( I am sure you do), but if you dont, then you can do this always, using Enterprise manager or SSMS object Explorer, create a new Identity column in the table and then create a unique composite key on these two field. ( You can create an identity column if you have data in the table, you DONT have to drop and recreate the table, using EM or SSMS you can always create a identity column in the table).

If you consider case a), then make sure there are no duplicates for the combination of new field and the field you chose for unique key.

If you create a composite unique key on cola and colb combined then you can have more than one null in cola and in colb, but again the combination of cola and colb should be unique.

Once you start making a composite unique key… this concept will be more easier.

Try doing this, either in Enterprise Manager or SSMS ( Object Explorer) much easier in interface than executing scripts.

Hope this helps.
Imran.

Thankx for your reply.

Your explanation is correct.

As mentioned by you, I am aware of SQL Server’s handling of various string data types.

In my opinion, ignoring trailing blanks while comparing string objects (aspecially when they are defined as variable length strings is a bad design flow).

I have designed a data warehouse where we are bringing data from Oracle whose table’s have key values e.g. AB and AB. Again having space in a key field itself is a bad design but structurally Oracle treats AB and AB as different values (which infact they are) and handles them OK.

When I bring then over to SQL server, I get unique key violation … vola … :)

My work around is to replace trailing blanks in key fields with ~ character. Again a temporary fix till SQL Server provides a way to have sound string data type comparisons.

Again, Thankx for your reply.

Tushar,
In Sql Server, any trailing spaces are ignored when comparing strings. I assume this is so that if a field is defined “char(10)” and another is defined “varchar(10)” and you insert ‘a’ into each one, then CharField = VarCharField will evaluate to TRUE even though the char(10) field actually contains an ‘a’ followed by 9 spaces.

So in statements like “where RTrim(CharField) = RTrim(VarCharField)”, the RTrim is not necessary. Even Len(CharField) and Len(VarCharField) will return 1 for both. However, leading spaces *are* significant.

create table a(b nvarchar(5))
insert into a values (‘a’)
insert into a values (‘a ‘) — please note space at the end
create unique index a1 on a(b)

gives error:

Msg 1505, Level 16, State 1, Line 2
The CREATE UNIQUE INDEX statement terminated because a duplicate key was found for the object name ‘dbo.a’ and the index name ‘a1′. The duplicate key value is (a ).

U can define unique key on varchar datatype but u can’t do it on bit datatype bcz bit datatype has values 0,1,null .

No u can’t use like that . bcz F.K is only reference to the P.K. Yes P.K is unique key but unique is not P.K.

the one more difference betn unique index and unique constraint is index is used to fetch the data fast. tht is use for improve the performance of fetching the data. where unique constraint is used to prevent the entering the redudant data in to the table.

When P.K is created at tht time cluster index is created autmatically and when unique key is created at that time non clustr index is created.

kind regards,

Azim

TABLE_B:
bcompno, bprdno (Refers cmpno, prdno of TABLE_A)

Can I have a Foreign Key relationship between TABLE_B and TABLE_A, using Unique Key, in SQL Server 2005?

Thanks in advance

FYI,

In fact, unique key allows single nulls.

E.g.

ID-Name(AS UNIQUE KEY)
1-Null
2-Joyal
3-Abhay

Thanks.

Regards,
Joyal

Plz tell me what is the difference between unique key and primary key

also tell me how could i download the small date bank and sql at home because i am learning it in germany and also working in a company plz tell me how could i free download .