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SQL FOREIGN KEY Constraint
May 13, 2010 This article explored the various types of constraints supported by DB2 for Linux, UNIX, and Windows, including the NOT NULL constraint, the unique constraint, the primary key constraint, the foreign key (referential) constraint, and table check constraints.
A FOREIGN KEY is a key used to link two tables together.
The implicit definition does not include any other attributes of the identified table or view. For example, the new table does not have a primary key or foreign key. The table is created in the table space implicitly or explicitly specified by the IN clause, and the table has any other optional clause only if the optional clause is specified. How to Find Primary Keys and Foreign Keys in a DB2 Table? How do I find primary keys and foreign keys in a DB2 table? I couldn't find them in systables or syscolumns of sysibm tables. Share this A. Asked Saturday May 31, 2003. Updated Wednesday March 06, 2019. SQL FOREIGN KEY Constraint. A FOREIGN KEY is a key used to link two tables together. 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. IBM DB2 Toad expert blog for developers, admins and data analysts. With solutions for Toad for Oracle, Toad for MySQL, Toad for SQL Server, DB2, SAP and more. Downloads Products Blog Forums Licenses. Toad World Blog View by Product. Toad® for Oracle Toad® for Oracle - Sensitive Data. For example, for tables, RazorSQL would look at column names, data types, sizes, primary keys, and foreign keys. The DDL generator may not be able to show all DDL for the particular object. For example, if there are check constraints or storage details on a table, RazorSQL may not. MySQL supports foreign keys, which let you cross-reference related data across tables, and foreign key constraints, which help keep this spread-out data consistent.The essential syntax for a foreign key constraint definition in a CREATE TABLE or ALTER TABLE statement looks like this.
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. Diablo 3 reaper of souls steam key generator.
Look at the following two tables:
'Persons' table:
| PersonID | LastName | FirstName | Age |
|---|---|---|---|
| 1 | Hansen | Ola | 30 |
| 2 | Svendson | Tove | 23 |
| 3 | Pettersen | Kari | 20 |
'Orders' table:
| OrderID | OrderNumber | PersonID |
|---|---|---|
| 1 | 77895 | 3 |
| 2 | 44678 | 3 |
| 3 | 22456 | 2 |
| 4 | 24562 | 1 |
Notice that the 'PersonID' column in the 'Orders' table points to the 'PersonID' column in the 'Persons' table.
The 'PersonID' column in the 'Persons' table is the PRIMARY KEY in the 'Persons' table.
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:
OrderID int NOT NULL,
OrderNumber int NOT NULL,
PersonID int,
PRIMARY KEY (OrderID),
FOREIGN KEY (PersonID) REFERENCES Persons(PersonID)
);
SQL Server / Oracle / MS Access:
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:
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:
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:
Db2 Foreign Key Definition Is Not Showing In Toad Generation Free
MySQL / SQL Server / Oracle / MS Access:
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:
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MySQL:
DROP FOREIGN KEY FK_PersonOrder;
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SQL Server / Oracle / MS Access:
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DROP CONSTRAINT FK_PersonOrder;