I'm having problems with adding another primary key to my table.
I have 3 columns:
Account ID (Identity)
EmailID
Data field
When I made the table I had this to make the Account ID and the Email ID unique.
PRIMARY KEY (AccountID, EmailID)
I thought that would make my emailid unique, but then after I tried inserting another row with the same emailid it went through.
So I thought I missed something out.
Now for my question:
IF, I had to use alter, how do I alter the table/PK Constraint to modify the EmailID field and make it Unique?
IF I decided to drop the table and made a new one, how do I make those two primary keys unique?
You may ALTER the table and add a new UNIQUE CONSTRAINT on the EmailID column.
-- This will create a constraint which enforces that the field EmailID
-- have unique values
ALTER TABLE Your_Table_Name
ADD CONSTRAINT unique_constraint_name UNIQUE (EmailID)
It's worth noting though, that altering the table to add this new unique constraint doesn't mean that you have to drop the other PRIMARY KEY constraint that you have added for the (AccountID, EmailID) pair. That is, of course, unless your business logic dictates it.
When you make the grouping of (AcountID, EmailID) the PRIMARY KEY it specifies that both the AcountID and EmailID participate in uniquely identifying each individual record in that table. So, that means that you could have the following records in the table:
AccountID | EmailID | Other Fields
----------------------------------------------------------
100 | user#company.com | ....
101 | user2#othermail.com | ....
100 | user_alternate#mail.com | ....
In the previous example it is possible to have two records with the same AccountID, and that is valid because the PRIMARY KEY specifies that only the (AccountID, EmailID) pair has to be unique - which it is. It makes no stipulation about AccountID being unique independently.
In conclusion, you probably want to add yet another UNIQUE constraint on AccountID. Or simply make the AccountID alone the PRIMARY KEY and then add a UNIQUE constraint on EmailID.
If both AccountID and EmailID are candidate keys then only one can be the PK the other one will need a unique constraint.
From the POV of SQL Server it doesn't matter which one you choose as the PK. Foreign Key's can reference either the PK or a unique constraint but given that the PK is the clustered index by default it probably makes sense to choose AccountID as this is presumably narrower and more stable.
It sounds like an incorrect Primary key. It's more likely that emailID is intended to be your natural key but for some reason (maybe a development standard in your organization?) you want to use a surrogate ID, AccountID but you still intend for both email ID and surrogate ID to both be unique and have a one to one relationship. If this is true then your primary key should be AccountID and you should place a unique constraint on EmailID.
If you were to recreate the table, it could look like this. I assumed EmailID was referencing an email table instead of being an email address.
CREATE TABLE dbo.AccountEmails
(
AccountID int not null identity(1,1),
EmailID int not null,
Data varchar(max) null,
constraint PK_AccountEmails PRIMARY KEY //this is a unique single column primary key
(
AccountID
),
constraint FK_AccountEmails_EmailID FOREIGN KEY dbo.Email(EmailID) ON //this makes sure EmailID exists in the Email table
(
EmailID
),
constraint UQ_AccountEmails_EmailID UNIQUE //unique single column unique constraint
(
EmailID
),
constraint UQ_AccountEmails_AccountID_EmailID UNIQUE //the combination of AccountID and EmailID is also unique
(
AccountID,
EmailID
)
)
Given the fact that AccountID and EmailID are both seperately unique, I'm not sure UQ_AccountEmails_AccountID_EmailID is really necessary.
Related
I'm using SQL Server 2014 to create and insert data into tables and came along a problem when it comes to populating a table with a foreign key constraint in it. I have a table user and and a table city which were created beforehand.
I used code to alter the user table to include a cityId foreign key from table city with this code:
ALTER TABLE [Schema].[user]
ADD cityId UNIQUEIDENTIFIER NOT NULL
CONSTRAINT usr_cid_fk
FOREIGN KEY (cityId) REFERENCES [Schema].[city] (cityId);
GO
Basically I modified the user table by adding a field called cityId which i made foreign key. now the problem is that when inserting data, in the first line
INSERT INTO [Schema].[user](name, surname, dob, gender, .. )
cityId cannot be found to be mapped. Dunno why it ain't showing. In the design view it is listed as a foreign key so there should be no problems.
Thanks a lot
try :
ALTER TABLE [Schema].[user]
ADD cityId NUMBER NOT NULL
CONSTRAINT usr_cid_fk
FOREIGN KEY (cityId) REFERENCES [Schema].[city] (cityId);
Note :
For ADD cityId UNIQUEIDENTIFIER NOT NULL
By the SQL standard, a foreign key must reference either the primary key or a unique key of the parent table. If the primary key has multiple columns, the foreign key must have the same number and order of columns. Therefore the foreign key references a unique row in the parent table; there can be no duplicates.
I am creating a table Brands with the following schema :
UserId
CarId
Brand
The UserId references the id of an user in the user table
The CarId references the id of a car in the car table
The only query that I will use is a search bases on these 2 columns, to get the corresponding brand.
So my question was about the constraint part, as I am a beginner, I would like to know which type of constraint to use (index, primary key, clustered or non clustered, on each field or on the 2 fields together) to have my query the more optimized possible.
This is my script right now :
IF NOT EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[Brands]') AND type in (N'U'))
BEGIN
CREATE TABLE [Brands]
(
UserId BIGINT NOT NULL
CONSTRAINT [FK_Brands_Users]
FOREIGN KEY REFERENCES [Users] (UserId),
CarId BIGINT NOT NULL
CONSTRAINT [FK_Brands_Cars]
FOREIGN KEY REFERENCES [Cars] (CarId),
Brand nvarchar(15),
);
END
GO
ALTER TABLE [Brands]
ADD CONSTRAINT [Unique_UserId_BrandId] UNIQUE(UserId, BrandId);
GO
I would create a primary key including both fields. When you define a primary key it automatically create a clustered index. Also your primary key has a unique constraint build in. Your table is now a heap the way you wrote it here above which is not good. You can additionally create an extra non-clustered index on CarId. Having an additional non-clustered index on UserId is not usefull I think. The column UserId can use the clustered index because it's the first field in the clustered index but I'm not sure about that.
I have 4 SQL tables: User, Student, Professor and Publication.
User has the common columns for any kind of user;
Student has columns specific for a student;
Professor has columns specific for a professor;
Publication is only for professors.
So I have:
create table dbo.[User] (
Id int identity not null
constraint PK_User_Id primary key clustered (Id),
-- Other user columns
)
create table dbo.Student (
UserId int not null
constraint PK_Student_UserId primary key clustered (Id),
-- Other student columns
)
create table dbo.Professor (
UserId int not null
constraint PK_Professor_Id primary key clustered (Id),
-- Other student columns
)
create table dbo.Publication (
Id int identity not null
constraint PK_Publication_Id primary key clustered (Id),
UserId int not null
-- Other student columns
)
alter table dbo.Student
add constraint FK_Student_UserId foreign key (UserId) references dbo.[User](Id);
alter table dbo.Professor
add constraint FK_Professor_UserId foreign key (UserId) references dbo.[User](Id);
alter table dbo.Publication
add constraint FK_Publication_UserId foreign key (UserId) references dbo.Professor(Id);
QUESTION
Should I have a column Id as PK in Professor and Student tables?
And make, for example, (Id, UserId) as the PK of Professor (Same for student)
Then Publication would reference Professor.Id and not Professor.UserId.
I am asking this because it sounds strange to have Publication to reference UserId from Professor table which can be confusing when I will have more tables.
Could someone please advice me on this?
In your current schema arrangement and without knowing your use cases (programmatically), one could make the argument that you don't need the Id identity columns for any of the extension tables. I assume this would be a 1 to 1 relationship to the User table anyway, so you'd at least want a unique constraint on the UserID columns, which you'd get by making it a PK anyway.
Things I like to consider are:
Can a professor ever become a different user ?
Is it possible for a professor to exist without an user ?
Is it possible for a single user to be two professors (multiple disciplines?)
If so, why wouldn't you give every professor an unique Id (ProfessorId), and only create a foreign key to the User table (UserId, you could call this UserFk).
In the publication table you can reference the professor by his/her id and call this one ProfessorFk. This way you create very efficient references between tables. The publication table than also gets a single PublicationId as primary key.
Correct me if i'm wrong, i don't know your use case. But it would seem reasonable that a professor can have multiple publications, but a publication can also be written by multiple professors ? This would mean you need an extra table between publication and professor for the n-n relationship.
About creating a professor key that is a combined key (Id, UserId). I personally dislike combined keys, if you want to reference this professor from your publication table you need two columns.
This would also suggest you can have multiple professors for the same user, if so, go for the single Id option.
This means i would create the following setup:
User Table
UserId
Student Table
StudentId
UserFk
Professor Table
ProfessorId
UserFk
ProfessorPublication Table
ProfessorFk
PublicationFk
Publication Table
PublicationId
So, it partly is based on what you want to be able to do with your data, and partly just your preference.
I have this table and I don't know how to normalize.
1st table name, address, landline no., mobile no., e-mail ad, amount, and registration number https://docs.google.com/file/d/0B99TeByt30n2dDVLVHV4dU1yRFE/edit
and in the second table will be their monthly pledges. https://docs.google.com/file/d/0B99TeByt30n2TVh4c1dmLTFYOWs/edit
PersonTable
PersonId int primary key
Name varchar(50)
Addresss varchar(50)
etc.
PledgeTable
PledgeId int primary key,
PersonId int foreign key references PersonTable(PersonId)
PledgeDate datetime
PledgeAmount decimal(10,2)
Ok.
You have to re engineer the whole idea.
I will give you a few concepts to understand what to do.
Keep in mind that we must NOT to have data duplicates.
So an array can be named Customer.
Customer can have a customer id column, named c_id for instance.
c_id must be defined as integer UNIQUE Auto_Increment NOT NULL
That means that it's an attribute that grants each customer its uniqueness.
A customer might have same name and same surname with another customer, but never same c_id.
Auto increment means that the first entry will be automatically numbered with c_id = 1, the next 2 etc etc.
Keep in mind that if you pass a record in the database using PHP, you enter null as value for c_id. Auto Incremet does the job then.
So you got a customer table and its first attribute that will be defined as the Primary key.
For example here is a small table:
CREATE TABLE customer
(c_id integer UNIQUE Auto_Increment NOT NULL,
c_name varchar(100),
c_surname varchar(100),
c_address varchar(100),
PRIMARY KEY (c_id)
);
You have to add all the attributes the customer has in the table.
That was just an example.
PRIMARY KEY (c_id) line in the end,
sets c_id as the primary key that distinguishes each record as unique.
Then you got another table.
A Pledge table.
CREATE TABLE pledge
(pl_id integer UNIQUE Auto_Increment NOT NULL,
pl_date date,
pl_price double(9,2),
pl_c_id integer,
PRIMARY KEY (pl_id),
FOREIGN KEY (pl_c_id) REFERENCES customer (c_id));
What is new here?
Line:
pl_c_id integer,
and line:
FOREIGN KEY (pl_c_id) REFERENCES customer (c_id)
What happens here, is that you create a column, that will contain an existing c_id!
That way, you make a reference to a customer, by using his/her UNIQUE c_id.
This is defined as integer, so it can fit the key.
pl_c_id MUST be the same type as the primary key of the other table. Ok?
Also SQL will know what key we refer to, by reading the line:
FOREIGN KEY (pl_c_id) REFERENCES customer (c_id)
In plain English that means:
pl_c_id can be filled with values already declared in a Primary Key of another Table, named "customer" that uses as primary key the column named "c_id".
Got it?
Now you got the flexibility to use ANY date.
That is more normalized than what you got.
Usually a 3NF (Third Normal Form) you will be ok.
Oh! Not to mention that you can Google "3NF" or "Third Normal Form" and get nice results for your reference.
;)
Cheers.
Edit: Made this reply more simple to understand.
Would it be possible in SQL Server 2008 to have a table created with 2 columns that are at the same time primary and foreign keys? If yes, how would such a code look like? I've searched and came up with nothing.
Sure, no problem:
CREATE TABLE dbo.[User]
(
Id int NOT NULL IDENTITY PRIMARY KEY,
Name nvarchar(1024) NOT NULL
);
CREATE TABLE [Group]
(
Id int NOT NULL IDENTITY PRIMARY KEY,
Name nvarchar(1024) NOT NULL
);
CREATE TABLE [UserToGroup]
(
UserId int NOT NULL,
GroupId int NOT NULL,
PRIMARY KEY CLUSTERED ( UserId, GroupId ),
FOREIGN KEY ( UserId ) REFERENCES [User] ( Id ) ON UPDATE NO ACTION ON DELETE CASCADE,
FOREIGN KEY ( GroupId ) REFERENCES [Group] ( Id ) ON UPDATE NO ACTION ON DELETE CASCADE
);
This is quite commonly used to model many-to-many relations.
These are totally different constructs.
A Primary Key is used to enforce uniqueness within a table, and be a unique identifier for a certain record.
A Foreign Key is used for referential integrity, to make sure that a value exists in another table.
The Foreign key needs to reference the primary key in another table.
If you want to have a foreign key that is also unique, you could make a FK constraint and add a unique index/constraint to that same field.
For reference purposes, SQL Server allows a FK to refer to a UNIQUE CONSTRAINT as well as to a PRIMARY KEY field.
It is probably not a good idea since often you want to allow duplicate foreign keys in the table. Even if you don't now, in the future, you might, so best not to do this. See Is it fine to have foreign key as primary key?
Just a quick note - from Microsoft pages (http://msdn.microsoft.com/en-us/library/ms189049.aspx)...
"A foreign key constraint does not have to be linked only to a primary key constraint in another table; it can also be defined to reference the columns of a UNIQUE constraint in another table."
Not used often, but useful in some circumstances.