I have two tables that have a junction table in between due to fact that there is many to many relationship between. How do I make a foreign key reference from one of the tables to the junction table to obtain a specific ID(the primary key ID of the opposite table), the ID is part of the primary key of the junction table that is made up of two foreign keys of the two tables. I made a foreign key reference straight to the junction table but its throwing an error "The number of columns in the referencing column list for foreign key does not match those of the primary key in the referenced table" when launching the CREATE query for the tables. What should I do to successfully make a foreign key reference to junction table?
The many side of both relationships will be at the junction table eg (written in SQL)
create table t1 (
id int primary key
) ;
create table t2 (
id int primary key
) ;
create table junction (
t1id int references t1( id ) not null
, t2id int references t2( id ) not null
) ;
-- PK: 2 columns
alter table junction
add constraint pkey_j primary key( t1id, t2id ) ;
DBfiddle here.
Related
For example let's have the following table definition:
CREATE TABLE table1
(
id INT UNIQUE,
name VARCHAR(100) UNIQUE,
description VARCHAR(100),
PRIMARY KEY (id, name)
);
Now I would like to create another table which would have a foreign key to the above composite primary key. Would the following two statements be equivalent?
1)
CREATE TABLE table2
(
id INT PRIMARY KEY,
table1_id INT,
table1_name VARCHAR(100),
FOREIGN KEY (table1_id) REFERENCES table1(id),
FOREIGN KEY (table1_name) REFERENCES table1(name)
);
2)
CREATE TABLE table2
(
id INT PRIMARY KEY,
table1_id INT,
table1_name VARCHAR(100),
FOREIGN KEY (table1_id, table1_name) REFERENCES table1(id, name),
);
I noticed that behind the scenes Postgre SQL creates two FK db objects in the case of 1) and one FK object in the case of 2). Would everything work the same anyway?
Not at all. A foreign key reference should be to the primary key. In this case you have a composite primary key (although that is probably not needed, see below). Although foreign key references to unique keys are allowed (and some databases even allow foreign key references to any indexed columns), that is not a best practice.
When you use a composite primary key (your second example) you are guaranteeing that id/name are aligned in the first table. When you use separate references (your first example), you do not know that they are aligned, so the id could refer to one row in table1 and the name to another row. I doubt that is your intention.
In any case, repeating redundant data among tables is a bad practice. The better data model is:
CREATE TABLE table1 (
id INT PRIMARY KEY,
name VARCHAR(100) UNIQUE,
description VARCHAR(100),
);
CREATE TABLE table2 (
id INT PRIMARY KEY,
table1_id INT,
FOREIGN KEY (table1_id) REFERENCES table1(id)
);
Then, if you want the corresponding name, look up the name in the first table.
As a note, in Postgres, I would expect the INT to really be SERIAL so the database assigns a unique, increasing value when you insert new rows.
If you actually want two references to table1 then use two id references:
CREATE TABLE table2 (
id INT PRIMARY KEY,
table1_id INT,
table1_id_2 INT,
FOREIGN KEY (table1_id) REFERENCES table1(id),
FOREIGN KEY (table1_id_2) REFERENCES table1(id)
);
A similar question is asked here multiple foreign keys referencing single column in other table
but the syntax is not shown in the answer. I would like to know how this can be accomplished in SQL server. The following syntax gives error
ALTER TABLE ItemIssue ADD CONSTRAINT FK_ItemIssue_Person
FOREIGN KEY (PersonID, AdvisorID) REFERENCES Person (PersonID)
;
ERROR: Number of referencing columns in foreign key differs from number of referenced columns, table 'ItemIssue'.
-- Create Tables
CREATE TABLE ItemIssue (
ItemIssueID int identity(1,1) NOT NULL,
PersonID int,
AdvisorID int,
)
;
CREATE TABLE Person (
PersonID int NOT NULL,
Name nvarchar(500),
)
;
You need to define two foreign keys, one for each column:
ALTER TABLE ItemIssue ADD CONSTRAINT FK_ItemIssue_Person
FOREIGN KEY (PersonID) REFERENCES Person (PersonID)
;
ALTER TABLE ItemIssue ADD CONSTRAINT FK_ItemAdvisor_Person
FOREIGN KEY (AdvisorID) REFERENCES Person (PersonID)
;
It is impossible to create one foreign key for two columns referencing one column. Create them seperate:
ALTER TABLE ItemIssue
ADD CONSTRAINT FK_ItemIssue_Person_Person FOREIGN KEY (PersonID) REFERENCES Person (PersonID),
ADD CONSTRAINT FK_ItemIssue_Advisor_Person FOREIGN KEY (AdvisorID) REFERENCES Person (PersonID);
To define two foreign keys, one for each column-
Table
Contract - HospidPharmacyId Column
Hospice- HospiceID PK
Pharmacy PharmacyId Pk
Using Following Query we can apply 2 Foreign Key for 1 column.
Alter Table Contract
Add Constraint fk_pharmacyID Foreign Key ([HospIDPharmID]) references Pharmacy([PharmacyID])
Alter TAble contract
Add Constraint Fk_hospId Foreign key ([HospIDPharmID]) references Hospice(HospiceID)
In the Contract Table for column-HospidPharmacyId we can insert common value in both the
tables. those which are present in hospice & not in Pharmacy then we cant insert that value in
contract table & vice versa.
I have 2 tables both with primary ids:
CREATE TABLE Table1
( Id INT NOT NULL AUTO_INCREMENT,
CONSTRAINT t1_pkey PRIMARY KEY (Id));
CREATE TABLE Table2
( Id INT NOT NULL AUTO_INCREMENT,
CONSTRAINT t2_pkey PRIMARY KEY (Id));
I have a third table which I am trying to setup a foreign key
CREATE TABLE Action
( TableId INT NOT NULL AUTO_INCREMENT,
CONSTRAINT ac_pkey PRIMARY KEY (Id));
I need to add a foreign key that can reference either table1 or table2 depending on which one has value. Is this possible or am I going to have to setup a parent table for tables 1 and 2?
It is not possible for a foreign key to reference one table or the other.
You could combine table1 and table2 with a type column and then have the combination of id, type be the primary key of the combined table and the foreign key in Action.
You could create a new table that is a parent of both table1 and table2 as well as Action
You could create two separate columns in Action, on that references table1 and the other that references table2 and then create a check constraint that ensures that only one of those is populated.
Which approach you prefer will come down to exactly what you're trying to model.
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've a sql table with a primary key(Auto Incremented) and a foreign key.Now I need to modify the table by modifying the foreign key to second primary key so that its values are not allowed to duplicate.
How do i alter my table without affecting the data? Need the sql code.
Regards,
Vix
If I understand your request, you want to force the foreign key to be unique within the given table so your schema looks like:
Create Table Table1
(
Id int not null primary key clustered
, ForeignId not null
, ...
, Constraint FK_Table1_Table2
Foreign Key ( ForeignId )
References Table2( Id )
)
And you now want to force ForeignId to be unique in this table, correct? You would do the following:
Alter Table Table1
Add Constraint UC_Table1_ForeignId Unique Nonclustered ( ForeignId )