![]() ![]() ![]() HINT: Truncate table "order_items" at the same time, or use TRUNCATE. You'll get notice ERROR: cannot truncate a table referenced in a foreign key constraintĭETAIL: Table "order_items" references "orders". Suppose you had two tables orders and order_items where the order_items table references the orders.order_id column. CASCADE construct which was introduced in PostgreSQL 8.2, which will not only delete all data from the main table, but will CASCADE to all the referenced tables. In comes its extended form, the TRUNCATE TABLE. You however can't use TRUNCATE TABLE unqualified, if the table you are truncating has foreign key references from other tables. Our preferred is the TRUNCATE TABLE approach because it's orders of magnitude faster than the DELETE FROM construct. In Postgres (and other RDBMs) cascading updates apply exclusively to foreign keys. Let's further illustrate the difference with an example.Though it is a rare occurrence, we have had occasions where we need to purge ALL data from a table. If you prefer to defer the check until the end of the transaction, use NO ACTION INITIALLY DEFERRED. NO ACTION is the default behavior if you do not specify anything. ![]() In practice, you can use either NO ACTION or RESTRICT depending on your needs. Just like RESTRICT, the database will not delete, update or set to NULL any rows in the referenced table(s). Otherwise if there are still rows referencing the parent row by the end of the transaction, an error will be raised just like before. If, for example there is another foreign key constraint between the same tables marked as CASCADE, the cascade will occur first and delete the referenced rows, and no error will be thrown by the deferred constraint. The difference from RESTRICT is that a constraint marked as NO ACTION INITIALLY DEFERRED is deferred until the end of the transaction, rather than running immediately. This will only raise the above error if the referenced rows still exist at the end of the transaction. However unlike RESTRICT, NO ACTION has the option defer the check using INITIALLY DEFERRED. When a foreign key constraint is defined with the option NO ACTION, it means that if a row in the parent table is deleted, the database will also raise an error and prevent the deletion of the row in the parent table. ![]() Delete any rows referencing the deleted row, or update the values of the. The database will not delete, update or set to NULL any rows in the referenced table(s). pgAdmin - PostgreSQL Tools for Windows, Mac, Linux and the Web. The name will be displayed in the pgAdmin tree control. Use the Name field to add a descriptive name for the foreign key. The SQL tab displays the SQL code generated by dialog selections. When a foreign key constraint is defined with the option RESTRICT, it means that if a row in the parent table is deleted, the database will immediately raise an error and prevent the deletion of the row in the parent table. The Foreign key dialog organizes the development of a foreign key constraint through the following dialog tabs: General, Definition, Columns, and Action. However, there is a subtle difference in how they behave. The difference between NO ACTION and RESTRICT is subtle and can be a bit confusing.īoth NO ACTION and RESTRICT are used to prevent deletion of a row in a parent table if there are related rows in a child table. This means that when a row is deleted from the "parent_table", all related rows in the "child_table" will be deleted as well. For example, the following SQL statement creates a foreign key constraint with the CASCADE option: These options can be specified when defining a foreign key constraint using the "ON DELETE" clause. This means that other cascading deletes can run first, and then this delete constraint will only throw an error if there is referenced data remaining at the end of the transaction. NO ACTION: This option is similar to RESTRICT, but it also has the option to be “deferred” to the end of a transaction.SET DEFAULT: When a row is deleted from the parent table, the values of the foreign key columns in the child table(s) are set to their default values.SET NULL: When a row is deleted from the parent table, the values of the foreign key columns in the child table(s) are set to NULL.RESTRICT: When a row is deleted from the parent table, the delete operation is aborted if there are any related rows in the child table(s).CASCADE: When a row is deleted from the parent table, all related rows in the child table(s) are deleted as well.There are 5 options for foreign key constraint deletes: ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |