I have two tables Users & Stores
Users table primary key is combination of address & phone number (users enroll through a web page and I don't want the same user to enroll twice), the userId column is serial but not a primary key
In Stores table the column of ownerID is it's userID from the users table - but since it's not primary key the reference can not be set (although it's serial)
how can I achieve this result?
Referenced column need not to be a primary key.
A foreign key can reference columns that either are a primary key OR a unique constraint.
This can be done in this way:
CREATE TABLE Users(
address varchar(100),
phone_number varchar(20),
userid serial,
constraint pk primary key (address, phone_number ),
constraint userid_unq unique (userid)
);
create table Stores(
storeid int primary key,
ownerID integer,
constraint b_fk foreign key (ownerID)
references Users(userid)
);
You should make userid primary key, so you could reference it easily in foreign keys. To eliminate duplication of phone and address, you can define a unique constraint or unique index for those columns.
Related
I'm creating a SQL table in VS that stores what rooms each client is, So the table has RoomId (int) and UserID (int).
Because I only want to add to the table only rooms and clients that exist they are both keys that have a foreign key to 2 tables, one that stores RoomID and Name and another that stores Client ID and Name.
Room and UserId tables:
CREATE TABLE [dbo].[UsersInRoomsTable]
(
RoomId INT NOT NULL,
UserId INT NOT NULL,
CONSTRAINT PK_RS PRIMARY KEY(RoomId, UserId),
CONSTRAINT [fk_room] FOREIGN KEY([RoomId]) REFERENCES [dbo].[RoomsTable]([RoomId]),
CONSTRAINT [fk_user] FOREIGN KEY ([UserId]) REFERENCES [dbo].[UserInfoTable] ([UserId])
);
Table that stores all the users:
CREATE TABLE [dbo].[UserInfoTable]
(
[UserName] NVARCHAR (50) NOT NULL,
[UserId] INT NOT NULL,
CONSTRAINT [PK_roomuser] PRIMARY KEY CLUSTERED ([UserName] ASC, [UserId] ASC)
);
Table that stores all the rooms
CREATE TABLE [dbo].[RoomsTable]
(
[RoomId] INT NOT NULL,
[RoomName] NVARCHAR (50) NOT NULL,
PRIMARY KEY CLUSTERED ([RoomId] ASC)
);
Everything works except the last line in the Rooms and users table:
CONSTRAINT [fk_user] FOREIGN KEY ([UserId]) REFERENCES [dbo].[UserInfoTable] ([UserId])
When I try to Update the table I get an error SQL71516:
SQL71516: The referenced table '[dbo].[UserInfoTable]' contains no primary or candidate keys that match the referencing column list in the foreign key.
If the referenced column is a computed column, it should be persisted
How can I solve this problem and what is causing it?
Edit: I think I know what is code is colliding: For some reason I can not have a foreign key connecting to a key that contains 2 indexes IE: fk_user is a Fk to table UserInfoTable that has 2 keys (UserID and UserName)
is there a way to pass this obstacle?
The columns of a foreign key have to match the columns they reference by number, type and order.
You have a primary key on userinfotable of (username, userid). But in usersinroomstable you are trying to let the foreign key (userid) to reference that. The number of columns doesn't match, so the foreign key cannot be added.
Presumably the username shouldn't really be part of the primary key of userinfotable and got there by accident. Remove it from the primary key constraint.
Or, if username has to be in the primary key, add such a column to the table usersinroomstable and add it to the foreign key constraint.
I want to know what is the difference between unique key and composite primary key in SQL Server.
According to w3c school:
The UNIQUE constraint uniquely identifies each record in a database table.
The UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a column or set of columns.
A PRIMARY KEY constraint automatically has a UNIQUE constraint defined on it.
Note that you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.
We can create composite primary key by using this:
CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
UNIQUE (P_Id)
)
For composite primary key syntax:
CREATE TABLE Persons
(
P_Id int,
C_Id int,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
Primary Key (P_Id,C_Id)
);
The UNIQUE constraint uniquely identifies each record in a database table. This provide a guarantee for uniqueness for a column or set of columns. We can define(point) a single row with this.
A PRIMARY KEY has a UNIQUE constraint by default.
While in some tables, there won't be any columns with a unique value to define a row. In such cases COMPOSITE KEYs are used. In such cases two or more columns are combined together so that this combination is unique.
As you said yourself, the only difference between a unique and a primary key is that there may only be 1 primary key on a table while it can have more unique keys.
Furthermore, the values of a primary key may not be null.
Both unique- and primary keys can be composed of multiple columns (composed key).
By the ways, in your example you had a unique key on the P_Id column and a composed primary key that includes that very same column.
This has no sense.
I would suggest to create only a simple primary key on that P_Id column.
Composite keys are formed of more than one column
Primary key prevents a second row on that table with same key column values.
A Primary key can be formed of a single column and as well as of more than one column. So primary key can be defined as a composite key too.
The primary key does not accept the any duplicate and NULL values.
A primary key of one table can be referenced by foreign key of another table. A table can have more than one unique key unlike primary key. Unique key constraints can accept only one NULL value for column. Unique constraints are also referenced by the foreign key of another table. A composite key is having two or more attributes that together can uniquely identify a tuple in a table. Such a key is also known as Compound Key, where each attribute creating a key is a foreign key in its own right.
For example
create table Equipment (
Equipmenttype nvarchar(1,
Description nvarchar(10),
Primary Key (Equipmenttype)
)
Create table Room (
RoomID nvarchar(8),
Capacity numeric(3),
Roomtype(fk,nvarchar(2)
)
I want to create the following table...
create table RoomEquipment(
(RoomID here)
(Equipmenttype here)
but Equipmenttype is not a primary key.
You can create foreign key on columns that are PK or Unuque:
So you have to create a unique index on RoomID:
CREATE UNIQUE INDEX UI_Room_RoomID ON dbo.Room(RoomID);
Now you can create a foreign key in table RoomEquipment.
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.
Source: Foreign Key to non-primary key
Equipmenttype is a primary key, in its own table Equipment, and RoomID should be a primary key for Room.
So RoomEquipment is a standard many-to-many relationship table.
I have two tables:
Article
Subscription
In the Article table I have two columns that make up the primary key: id, sl. In the Subscription table I have a foreign key 'idsl`.
I use this constraint :
constraint FK_idsl
foreign key (idsl) references CSS_SubscriptionGroup(id, sl)
But when I run the query, I getting this error:
Number of referencing columns in foreign key differs from number of referenced columns, table X
In Article Table I have two fields that are the primary key: id,sl. In the Subscription Table I have a foreign key 'idsl`
This design is broken - it is apparent that the composite primary key in Article(id, sl) has been mangled into a single compound foreign key in table Subscription. This isn't a good idea.
Instead, you will need to change the design of table Subscription to include separate columns for both id and sl, of the same type as the Article Table, and then create a composite foreign key consisting of both columns, referencing Article in the same order as the primary key, e.g:
CREATE TABLE Article
(
id INT NOT NULL,
sl VARCHAR(50) NOT NULL,
-- Other Columns
CONSTRAINT PK_Article PRIMARY KEY(id, sl) -- composite primary key
);
CREATE TABLE Subscription
(
-- Other columns
id INT NOT NULL, -- Same type as Article.id
sl VARCHAR(50) NOT NULL, -- Same type as Article.sl
CONSTRAINT FK_Subscription_Article FOREIGN KEY(id, sl)
REFERENCES Article(id, sl) -- Same order as Article PK
);
Edit
One thing to consider here is that by convention a column named table.id or table.tableid should be unique, and is the primary key for the table. However, since table Article requires an additional column sl in the primary key, it implies that id isn't unique.
correct syntax for relation:
CONSTRAINT FK_OtherTable_ParentTable
FOREIGN KEY(OrderId, CompanyId) REFERENCES dbo.ParentTable(OrderId, CompanyId)
You must try like this:
constraint FK_idsl foreign key (id,sl) references CSS_SubscriptionGroup(id,sl)
I created a table like this:
CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
PRIMARY KEY (P_Id)
)
but I can't understand the purpose of the primary key
PRIMARY KEY (P_Id)
in that table.
From MSDN:
A table typically has a column or combination of columns that contain
values that uniquely identify each row in the table. This column, or
columns, is called the primary key (PK) of the table and enforces the
entity integrity of the table. You can create a primary key by
defining a PRIMARY KEY constraint when you create or modify a table.