Sqlite foreign key cascade example how to#
Sqlite foreign key cascade example code#

Post.BlogId is a foreign key property, the value of which must match the Blog.Id primary key of the blog to which the post belongs. Deleting a principal/parentĬonsider this simple model where Blog is the principal/parent in a relationship with Post, which is the dependent/child. This can happen because the principal/parent is deleted, or it can happen when the principal/parent still exists but the dependent/child is no longer associated with it.

When cascading behaviors happenĬascading deletes are needed when a dependent/child entity can no longer be associated with its current principal/parent.

Sqlite foreign key cascade example code#

Now, let's demonstrate how the ON DELETE SET NULL works.You can run and debug into all the code in this document by downloading the sample code from GitHub. In this example, we've created a foreign key (with set null on delete) called fk_departments that references the departments table based on the department_id field. INSERT INTO employees SELECT * FROM _employees_old Now, let's add a foreign key with ON DELETE SET NULL to the employees table: PRAGMA foreign_keys=off ĪLTER TABLE employees RENAME TO _employees_old INSERT INTO employees VALUES (10001, 'Anderson', 'Dave', 999) INSERT INTO employees VALUES (10000, 'Smith', 'John', 30) INSERT INTO departments VALUES (999, 'Sales') You can vote up the ones you like or vote down the ones you don't like, and go to the original project or source file by following the links above each example. INSERT INTO departments VALUES (30, 'HR') The following are 30 code examples of alembic.op.createforeignkey (). Next, let's add some data to these tables: For example, ON DELETE CASCADE effectively. INSERT INTO table1 SELECT * FROM _table1_old įirst, let's start by creating our 2 tables ( departments and employees): The referential action CASCADE modifies the behavior of the (child) table itself where the word CASCADE is used. The syntax to add a foreign key with 'set null on delete' to an existing table in SQLite is: PRAGMA foreign_keys=off ĪLTER TABLE table1 RENAME TO _table1_old Instead you will need to rename the table, create a new table with the foreign key, and then copy the data into the new table. You can not use the ALTER TABLE statement to add a foreign key with 'set null on delete' in SQLite. configuring a RESTRICT action causes SQLite to return an error.

Sqlite foreign key cascade example how to#

How to Add a Foreign Key with 'Set NULL on Delete' to an Existing Table Sqlite INSERT INTO a FOREIGN KEY constraint failed error have appeared because of the. Then we've created a foreign key called fk_departments on the employees table that references the departments table based on the department_id field.īecause of the ON DELETE SET NULL, when a record in the departments table is deleted, all corresponding records in the employees table will have the department_id set to NULL. In this example, we've created a primary key on the departments table that consists of only one field - the department_id field. ( employee_id INTEGER PRIMARY KEY AUTOINCREMENT, ( department_id INTEGER PRIMARY KEY AUTOINCREMENT, Let's look at an example of how to create a foreign key with 'set null on delete' using the CREATE TABLE statement in SQLite. REFERENCES parent_table (column1, column2. All children of the parent row are killed (deleted), too. The syntax for creating a foreign key with 'set null on delete' using a CREATE TABLE statement in SQLite is: CREATE TABLE table_nameįOREIGN KEY (column1, column2. 6 Answers Sorted by: 218 If you like the Parent and Child terms and you feel they are easy to be remembered, you may like the translation of ON DELETE CASCADE to Leave No Orphans Which means that when a Parent row is deleted (killed), no orphan row should stay alive in the Child table.

How to Create a Foreign Key with 'Set Null on Delete' using a CREATE TABLE statement Syntax