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Foreign Keys

A foreign key is a field (or collection of fields) in one table that uniquely identifies a row of another table. In other words, a foreign key is a column or a combination of columns that is used to establish and enforce a link between the data in two tables.
 

Note: To create a foreign key constraint, it is necessary to have this privilege for both the referencing and referenced tables.

 

 

Foreign keys are created within the Foreign Key Properties dialog window. In order to open the dialog you should either

 

open the table in Table Editor and the Foreign Keys tab there;
press the Insert key or select the Add New Foreign Key... item from the popup menu (alternatively, you may use the corresponding link of the Navigation Bar)

or

select the table in the explorer tree and use the Create New Foreign Key popup menu item

or

select the table Foreign Keys node or any foreign key within the table in the explorer tree and use the Add New Foreign Key... popup menu item.

 

 

 

Foreign Keys are edited within the Foreign Key Properties dialog window. In order to open the dialog you should either

 

open the table in Table Editor and the Foreign Keys tab there;
press the Enter key or select the Edit Foreign Key item from the popup menu (alternatively, you may use the corresponding link of the Navigation Bar)

or

select the foreign key to edit in the explorer tree and use the Edit Foreign Key popup menu item.

       

You can change the name of the foreign key using the Rename Foreign Key dialog. To open the dialog you should either

 

select the foreign key to rename in the explorer tree;
select the Rename Foreign Key item from the popup menu

or

open the table in Table Editor and the Foreign Keys tab there;
select the foreign key to rename;
select the Rename Foreign Key item from the popup menu (alternatively, you may use the corresponding link of the Navigation Bar).

 

 

 

To drop the foreign key:

 

select the foreign key to drop in the explorer tree;
select the Drop Foreign Key item from the popup menu

or

open the table in Table Editor and the Foreign Keys tab there;
press the Delete key or select the Drop Foreign Key item from the popup menu (alternatively, you may use the corresponding link of the Navigation Bar)

 

and confirm dropping in the dialog window.

 

 

Set the Foreign Key Name, select Columns from the Available Fields list to include into the foreign key, select the Foreign Table Name from the drop-down list and its fields from the list to include, set other foreign key properties and apply the changes by clicking the OK button.

 

 

All the fields which are included into the Foreign Key must be included into indexes as well. See Indexes for details.

 

 

Set rules ON DELETE and ON UPDATE from the respective drop-down lists.

 

NO ACTION Produce an error indicating that the deletion or update will create a foreign key constraint violation. If the constraint is deferred this error will be produced at constraint check time if there still exist any referencing rows. This is the default action.

 

RESTRICT Produce an error indicating that the deletion or update would create a foreign key constraint violation. This is the same as NO ACTION except that the check is not deferrable.

 

CASCADE Delete any rows referencing the deleted row, or update the value of the referencing column to the new value of the referenced column, respectively.

 

SET NULL Set the referencing column(s) to null.

 

SET DEFAULT Set the referencing column(s) to their default values.

 

Match type

A value inserted into the referencing column(s) is matched against the values of the referenced table and referenced columns using the given match type.

Match Full will not allow one column of a multi-column foreign key to be null unless all foreign key columns are null.

 

Match Simple, which is also the default, allows some foreign key columns to be null while other parts of the foreign key are not null.

 

Deferrable

This controls whether the constraint can be deferred. A constraint that is not deferrable will be checked immediately after every command. Checking of foreign key constraints that are deferrable may be postponed until the end of the transaction.
 

Comment

The box allows you to set optional text describing the foreign key.



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