Generated Always As Foriegn Key

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SQL FOREIGN KEY Constraint

  1. Oracle Generated Always As
  2. Generated Always As Row Start
  3. Generated Always As Identity

A FOREIGN KEY is a key used to link two tables together.

Identity columns as primary keys. The SELECT from INSERT statement enables you to insert a row into a parent table with its primary key defined as a DB2-generated identity column, and retrieve the value of the primary or parent key. You can then use this generated value as a foreign key in a dependent table. FOREIGN KEY, ADD PRIMARY KEY, ADD UNIQUE, DROP PRIMARY KEY, DROP FOREIGN KEY, or DROP CONSTRAINT is specified. The data type of a column that is added to the table is a distinct type. A fullselect is specified. A column is defined as a security label column. A column is defined as ROWID GENERATED BY DEFAULT.

A FOREIGN KEY is a field (or collection of fields) in one table that refers to the PRIMARY KEY in another table.

The table containing the foreign key is called the child table, and the table containing the candidate key is called the referenced or parent table.

Look at the following two tables:

'Persons' table:

PersonIDLastNameFirstNameAge
1HansenOla30
2SvendsonTove23
3PettersenKari20

'Orders' table:

OrderIDOrderNumberPersonID
1778953
2446783
3224562
4245621

Notice that the 'PersonID' column in the 'Orders' table points to the 'PersonID' column in the 'Persons' table.

https://ameblo.jp/feifectdrivcas1984/entry-12632578304.html. The 'PersonID' column in the 'Persons' table is the PRIMARY KEY in the 'Persons' table.

Run 8 v2 registration key generator. The 'PersonID' column in the 'Orders' table is a FOREIGN KEY in the 'Orders' table.

The FOREIGN KEY constraint is used to prevent actions that would destroy links between tables.

The FOREIGN KEY constraint also prevents invalid data from being inserted into the foreign key column, because it has to be one of the values contained in the table it points to.

SQL FOREIGN KEY on CREATE TABLE

The following SQL creates a FOREIGN KEY on the 'PersonID' column when the 'Orders' table is created:

MySQL:

CREATE TABLE Orders (
OrderID int NOT NULL,
OrderNumber int NOT NULL,
PersonID int,
PRIMARY KEY (OrderID),
FOREIGN KEY (PersonID) REFERENCES Persons(PersonID)
);

SQL Server / Oracle / MS Access:

CREATE TABLE Orders (
OrderID int NOT NULL PRIMARY KEY,
OrderNumber int NOT NULL,
PersonID int FOREIGN KEY REFERENCES Persons(PersonID)
);

To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

CREATE TABLE Orders (
OrderID int NOT NULL,
OrderNumber int NOT NULL,
PersonID int,
PRIMARY KEY (OrderID),
CONSTRAINT FK_PersonOrder FOREIGN KEY (PersonID)
REFERENCES Persons(PersonID)
);

SQL FOREIGN KEY on ALTER TABLE

To create a FOREIGN KEY constraint on the 'PersonID' column when the 'Orders' table is already created, use the following SQL:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Orders
ADD FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);

To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Orders
ADD CONSTRAINT FK_PersonOrder
FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);

DROP a FOREIGN KEY Constraint

To drop a FOREIGN KEY constraint, use the following SQL:

Oracle Generated Always As

MySQL:

ALTER TABLE Orders
DROP FOREIGN KEY FK_PersonOrder;

SQL Server / Oracle / MS Access:

Generated Always As Row Start

ALTER TABLE Orders
DROP CONSTRAINT FK_PersonOrder;

Generated Always As Identity