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.
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)
);
For this question, I am referring to the specific case where you have table T, it has primary key K, but K is a foreign key. Is this valid? And how would you write it in SQL99?
All the other questions I've seen on here are just asking whether or not a primary key can be a foreign key for another table. That's not what I'm asking about. I'm asking about a table which has a foreign key where that is the primary key of that table.
If I understand you correctly, you want to create a hierarchical table, for instance:
create table hierarchical
(
id number primary key,
parent_id number
);
alter table hierarchical add constraint
fk_parent_id foreign key(parent_id) references hierarchical(id);
This kind of table can contain employees/managers for instance.
A column can be a primary key as well foreign key. For example, refer to the following:
A column can be both a primary key and a foreign key. For example:
create table A
(
id int not null
, constraint PK_A primary key (id)
);
create table B
(
id int not null
,constraint PK_B primary key (id)
,constraint FK_B_ID foreign key (id) references A(id)
);
Though, this requires data to be present in Table B first.
Let's suppose that there are two tables table1 and table2 where the second one references the first one.
I was wondering whether the following two forms of defining their foreign key relationship results in the same table structure of the second table.
CREATE TABLE table1(a INT, b INT, PRIMARY KEY(a, b));
1)
CREATE TABLE table2(a INT, b INT,
FOREIGN KEY(a, b) REFERENCES table1(a, b)
);
2)
CREATE TABLE table2(a INT, b INT,
FOREIGN KEY(a) REFERENCES table1(a),
FOREIGN KEY(b) REFERENCES table1(b)
);
I would presume that there is no difference but couldn't find any reference to support this claim.
First, this syntax is not correct:
CREATE TABLE table1 (
a INT PRIMARY KEY,
b INT PRIMARY KEY
);
A primary key is primary because only one can be defined. Presumably, you intend a composite primary key:
CREATE TABLE table1 (
a INT,
b INT,
PRIMARY KEY (a, b)
);
The foreign key relationships are entirely different. The second creates two foreign key relationships. With a composite primary key, it would fail, because the reference should be to a primary key (or at least a unique key).
This is my table 1:
CREATE TABLE PurchasedProducts
(
Purchase_Order_No int,
Purchase_Product_ID int FOREIGN KEY REFERENCES Inventory(In_Product_ID),
Purchase_Quantity int NOT NULL,
Purchase_Status varchar(7) NOT NULL,
PRIMARY KEY(Purchase_Order_No, Purchase_Product_ID)
);
This my table 2:
CREATE TABLE PurchasedDate
(
PD_PO_No int NOT NULL PRIMARY KEY FOREIGN KEY REFERENCES PurchasedProducts(Purchase_Order_No),
PD_Date date NOT NULL
);
I executed the first table successfully, but when I execute the second table It is showing this error message:
There are no primary or candidate keys in the referenced table 'PurchasedProducts' that match the referencing column list in the foreign key 'FK__Purchased__PD_PO__0B5CAFEA'.
I don't what is the problem is. Please help me!
The primary key in your PurchasedProducts table is made up of two columns:
PRIMARY KEY(Purchase_Order_No, Purchase_Product_ID)
So any child table that wants to reference that also must have these exact two columns:
CREATE TABLE PurchasedDate
(
PD_PO_No int NOT NULL PRIMARY KEY,
Purchase_Product_ID INT NOT NULL,
PD_Date date NOT NULL
);
ALTER TABLE dbo.PurchasedDate
ADD CONSTRAINT FK_PurchaseDate_PurchasedProducts
FOREIGN KEY(PD_PO_No, Purchase_Product_ID)
REFERENCES PurchasedProducts(Purchase_Order_No, Purchase_Product_ID)
A foreign key can only reference the whole primary key of a parent table - you cannot reference only one column out of 2 from the parent table's PK.
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