There is table A with columns (Id, BId). BId is foreign key to table B. B has columns (Id, Type).
CREATE TABLE [A] (
[Id] INT IDENTITY CONSTRAINT [PK_A_Id] PRIMARY KEY,
[BId] INT NOT NULL CONSTRAINT [FK_A_B] REFERENCES [B](Id)
)
GO
CREATE TABLE [B] (
[Id] INT IDENTITY CONSTRAINT [PK_B_Id] PRIMARY KEY,
[Type] INT NOT NULL
)
GO
So, it is very simple scheme, but I want to add condition for foreign keys like "type should be 0". It should be something like
CONSTRAINT [FK_A_B] REFERENCES [B](Id) WHERE [B].[Type] = 0
How to use UNIQUE keyword or smth else correctly to realize it?
What you are trying to achieve is not a task for FOREIGN KEY. In FK you can't specify additional conditions on a target table. If you have such need, usually it means that your DB is not normalized. It looks like table [B] stores several data entities in one table, and Type column determines what entity it is. If you broke normalization rules, declarative integrity means like FK don't work for you. From now on you have to control integrity on your own. You can do this in application logic or create triggers (procedural integrity). In any case there will be no foreign key constraint in DB.
It would be nice if this could be done using a filtered unique index. But, SQL Server doesn't allow that. So here is another idea:
Start by defining a (redundant) unique constraint on:
CREATE TABLE [B] (
[Id] INT IDENTITY CONSTRAINT [PK_B_Id] PRIMARY KEY,
[Type] INT NOT NULL,
UNIQUE (ID, Type)
)
GO
Now, I don't think you can define the constraint as:
CONSTRAINT [FK_A_B] (ID, 0) REFERENCES [B](Id, Type)
But, I think you can trick it by doing:
_Type as 0,
CONSTRAINT [FK_A_B] (ID, _Type) REFERENCES [B](Id, Type)
That is, add a computed column and use that for the constraint.
Related
This is my first Table : Tours
CREATE TABLE [dbo].[Tours] (
[Id] INT IDENTITY (1, 1) NOT NULL,
[TourId] INT NOT NULL,
[TCountry] NVARCHAR (50) NULL,
[TName] NVARCHAR (100) NULL,
CONSTRAINT [PK_Tours] PRIMARY KEY CLUSTERED ([TourId]),
CONSTRAINT [FK_Tours_ToTourDates] FOREIGN KEY ([TourId]) REFERENCES [TourDates]([TourId]));
and this is the next, Table:TourDates
CREATE TABLE [dbo].[TourDates] (
[Id] INT IDENTITY (1, 1) NOT NULL,
[TourId] INT NOT NULL,
[TourStartDate] DATETIME NULL,
CONSTRAINT [PK_TourDates] PRIMARY KEY CLUSTERED ([Id] ASC));
for first table I have this Error :
SQL71516 :: The referenced table '[dbo].[TourDates]' 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 one define a Candidate key for TourId column in TourDates Table??
I believe that you should be doing something else than you are doing right now.
Tours table seems to hold every tour which should be in 1:N relationship with TourDates provided that I understand it correctly.
Thus, your FOREIGN KEY constraint should actually be declared on TourDates, not Tours table.
As to your issue (which I believe wouldn't if you switch those relationships to what they should be in my understanding) in such cases you would normally need to create a unique index on that column.
You can't reference a column which may contain several exact same values via foreign key constraint, thus the need for some sort of a unique key.
Edit after comment:
ALTER TABLE [dbo].[TourDates]
ADD CONSTRAINT FK_Tour_TourDates
FOREIGN KEY ([TourId]) REFERENCES [dbo].[Tours]([TourId]) ON DELETE CASCADE
Table A
(
Table_A_ID int
)
Table B
(
Table_B_ID int
Value int
)
Say I want to insert data into Table B, where 'Value' would be the same as a Table_A_ID.
How would I make a constraint or check that the data actually exists in the table on insertion?
You probably need to enforce data integrity not only on INSERT into Table B, but also on UPDATE and DELETE in both tables.
Anyway options are:
FOREIGN KEY CONSTRAINT on Table B
TRIGGERs on both tables
As a last resort if for some reason 1 and 2 is not an option STORED PROCEDUREs for all insert, delete update operations for both tables
The preferred way to go in most cases is FOREIGN KEY CONSTRAINT.
Yap, I agree with #peterm.
Cause, if your both Table_A_ID and Table_B_Id are primary keys for both tables, then you don't even need two tables to store the value. Since, your two tables are seems to be on 'one-to-one' relationship. It's one of the database integrity issues.
I think you didn't do proper normalisation for this database.
Just suggesting a good idea!
I found this example which demonstrates how to setup a foreign key constraint.
Create employee table
CREATE TABLE employee (
id smallint(5) unsigned NOT NULL,
firstname varchar(30),
lastname varchar(30),
birthdate date,
PRIMARY KEY (id),
KEY idx_lastname (lastname)
) ENGINE=InnoDB;
Create borrowed table
CREATE TABLE borrowed (
ref int(10) unsigned NOT NULL auto_increment,
employeeid smallint(5) unsigned NOT NULL,
book varchar(50),
PRIMARY KEY (ref)
) ENGINE=InnoDB;
Add a constraint to borrowed table
ALTER TABLE borrowed
ADD CONSTRAINT FK_borrowed
FOREIGN KEY (employeeid) REFERENCES employee(id)
ON UPDATE CASCADE
ON DELETE CASCADE;
NOTE: This tells MySQL that we want to alter the borrowed table by adding a constraint called ‘FK_borrowed’. The employeeid column will reference the id column in the employee table – in other words, an employee must exist before they can borrow a book.
The final two lines are perhaps the most interesting. They state that if an employee ID is updated or an employee is deleted, the changes should be applied to the borrowed table.
NOTE: See the above URL for more details, this is just an excerpt from that article!
Create a foreign key constraint on the column 'Value' on table B that references the 'Table_A_ID' column.
Doing this will only allow values that exist in table A to be added into the 'Value' field of table B.
To accomplish this you first need to make Table_A_ID column the primary key for table A, or it at least has to have some sort of unique constraint applied to it to be a foreign key candidate.
BEGIN TRANSACTION -- REMOVE TRANSACTION AND ROLLBACK AFTER DONE TESTING
--PUT A PRIMARY KEY ON TABLE A
CREATE TABLE A
( Table_A_ID int CONSTRAINT PK_A_Table_A_ID PRIMARY KEY)
--ON VALUE ADD A FOREIGN KEY CONSTRAINT THAT REFERENCEs TABLE A
CREATE TABLE B
( Table_B_ID int,
[Value] int CONSTRAINT FK_B_Value_A REFERENCES A(Table_A_ID)
)
-- TEST VALID INSERT
INSERT A (Table_A_ID) VALUES (1)
INSERT B (Table_B_ID, [Value]) VALUES (1,1)
--NOT ALLOW TO INSERT A VALUE THAT DOES NOT EXIST IN A
--THIS WILL THROW A FOREIGN KEY CONSTRAINT ERROR
INSERT B (Table_B_ID, [Value]) VALUES (1,2) -- 2 DNE in table A
ROLLBACK
Note: there is no magic to 'FK_B_Value_A' or 'PK_A_Table_A_ID' it simply a naming convention and be called anything. The syntax on the foreign key and primary key lines work like this:
column-definition CONSTRAINT give-the-constraint-a-name REFERENCES table-name ( table-column )
column-definition CONSTRAINT give-the-constraint-a-name PRIMARY KEY
I want to create tables in SQL Server 2008, but I don't know how to create composite primary key. How can I achieve this?
create table my_table (
column_a integer not null,
column_b integer not null,
column_c varchar(50),
primary key (column_a, column_b)
);
CREATE TABLE UserGroup
(
[User_Id] INT NOT NULL,
[Group_Id] INT NOT NULL
CONSTRAINT PK_UserGroup PRIMARY KEY NONCLUSTERED ([User_Id], [Group_Id])
)
Via Enterprise Manager (SSMS)...
Right Click on the Table you wish to create the composite key on and select Design.
Highlight the columns you wish to form as a composite key
Right Click over those columns and Set Primary Key
To see the SQL you can then right click on the Table > Script Table As > Create To
I know I'm late to this party, but for an existing table, try:
ALTER table TABLE_NAME
ADD CONSTRAINT [name of your PK, e.g. PK_TableName] PRIMARY KEY CLUSTERED (column1, column2, etc.)
For MSSQL Server 2012
CREATE TABLE usrgroup(
usr_id int FOREIGN KEY REFERENCES users(id),
grp_id int FOREIGN KEY REFERENCES groups(id),
PRIMARY KEY (usr_id, grp_id)
)
UPDATE
I should add !
If you want to add foreign / primary keys altering, firstly you should create the keys with constraints or you can not make changes. Like this below:
CREATE TABLE usrgroup(
usr_id int,
grp_id int,
CONSTRAINT FK_usrgroup_usrid FOREIGN KEY (usr_id) REFERENCES users(id),
CONSTRAINT FK_usrgroup_groupid FOREIGN KEY (grp_id) REFERENCES groups(id),
CONSTRAINT PK_usrgroup PRIMARY KEY (usr_id,grp_id)
)
Actually last way is healthier and serial. You can look the FK/PK Constraint names (dbo.dbname > Keys > ..) but if you do not use a constraint, MSSQL auto-creates random FK/PK names. You will need to look at every change (alter table) you need.
I recommend that you set a standard for yourself; the constraint should be defined according to the your standard. You will not have to memorize and you will not have to think too long. In short, you work faster.
First create the database and table, manually adding the columns. In which column to be primary key. You should right click this column and set primary key and set the seed value of the primary key.
To create a composite unique key on table
ALTER TABLE [TableName] ADD UNIQUE ([Column1], [Column2], [column3]);
CREATE TABLE UserGroup
(
[User_Id] INT Foreign Key,
[Group_Id] INT foreign key,
PRIMARY KEY ([User_Id], [Group_Id])
)
create table d(id numeric(1), code varchar(2))
After I create the above table how can I add a composite primary key on both fields and also a foreign key?
In Oracle, you could do this:
create table D (
ID numeric(1),
CODE varchar(2),
constraint PK_D primary key (ID, CODE)
);
alter table d add constraint pkc_Name primary key (id, code)
should do it. There's lots of options to a basic primary key/index depending on what DB your working with.
The ALTER TABLE statement presented by Chris should work, but first you need to declare the columns NOT NULL. All parts of a primary key need to be NOT NULL.
You don't need to create the table first and then add the keys in subsequent steps. You can add both primary key and foreign key while creating the table:
This example assumes the existence of a table (Codes) that we would want to reference with our foreign key.
CREATE TABLE d (
id [numeric](1),
code [varchar](2),
PRIMARY KEY (id, code),
CONSTRAINT fk_d_codes FOREIGN KEY (code) REFERENCES Codes (code)
)
If you don't have a table that we can reference, add one like this so that the example will work:
CREATE TABLE Codes (
Code [varchar](2) PRIMARY KEY
)
NOTE: you must have a table to reference before creating the foreign key.
If using Sql Server Management Studio Designer just select both rows (Shift+Click) and Set Primary Key.
Naive question with the answer "No" , I believe, but still would like to ask.
Table_parent
pk_parent_surrogate
parent_natural_unique_key
Table_child
pk_child_surrogate
child_natural_NOT_unique
Is that true that the only possible declarative relationship among main database vendors is
pk_parent_surrogate ----------< pk_child_surrogate
and we can't have declarative constraint or foreign key in other words for pair
parent_natural_unique_key -------< child_natural_NOT_unique
My answer here is based on my MS SQL knowledge - although I believe the same answer is correct for ANSI standards as well, i'm not 100% sure...
YES - you CAN do this as long as you've got a unique constraint on the column in your parent table that you want to use as the anchor column for the key.
You can create a FOREIGN KEY constraint as part
of the table definition when you create a table.
If a table already exists, you can add a
FOREIGN KEY constraint, provided that the
FOREIGN KEY constraint is linked to an existing
PRIMARY KEY constraints or UNIQUE constraint in
another, or the same, table. A table can contain
multiple FOREIGN KEY constraints.
And as an example of this sort of key...
use tempdb
CREATE TABLE parent(
pk int identity primary key,
candidate_key int unique not null)
CREATE TABLE child(
pk int identity primary key,
join_key int references parent(candidate_key))
See here for more information.
Try code like this:
create table testunique (id int identity(1,1) primary key, otherid int)
go
create unique index ixOther on testunique(otherid)
go
create table testFK (id int identity(1,1) primary key, someid int)
go
alter table testFK add constraint fkTest foreign key (someid) references testunique(otherid)
Rob