Idatabase keydelete how to#
Idatabase keydelete code#

We have used the CREATE TABLE statement to create a foreign key on the inventory table called fk_inv_product_id. Next, we've created a second table called inventory that will be the child table in this foreign key example. The products table has a primary key that consists of the product_id field.

In this foreign key example, we've created our parent table as the products table.

Idatabase keydelete how to#

Let's look at an example of how to create a foreign key with set null on delete in SQL Server (Transact-SQL) using the CREATE TABLE statement. It means that the child data is set to their default values when the parent data is deleted or updated. SET DEFAULT It is used in conjunction with ON DELETE or ON UPDATE.

It means that the child data is set to NULL when the parent data is deleted or updated. SET NULL It is used in conjunction with ON DELETE or ON UPDATE.

It means that the child data is either deleted or updated when the parent data is deleted or updated. CASCADE It is used in conjunction with ON DELETE or ON UPDATE. It means that no action is performed with the child data when the parent data is deleted or updated. NO ACTION It is used in conjunction with ON DELETE or ON UPDATE. You have the options of NO ACTION, CASCADE, SET NULL, or SET DEFAULT. It specifies what to do with the child data when the parent data is updated. ON DELETE SET NULL It specifies that the child data is set to NULL when the parent data is deleted. The foreign key will enforce a link between this data and the child_col1, child_col2. parent_col3 The columns that make up the primary key in the parent_table. parent_table The name of the parent table whose primary key will be used in the child_table. child_col_n The columns in child_table that will reference a primary key in the parent_table. fk_name The name of the foreign key constraint that you wish to create. The column should either be defined as NULL or NOT NULL and if this value is left blank, the database assumes NULL as the default.

column1, column2 The columns that you wish to create in the table. ) child_table The name of the child table that you wish to create. REFERENCES parent_table (parent_col1, parent_col2. The syntax for creating a foreign key with set null on delete using a CREATE TABLE statement in SQL Server (Transact-SQL) is: CREATE TABLE child_tableįOREIGN KEY (child_col1, child_col2.

Idatabase keydelete code#

Let’s modify our code and verify all of what we’ve just said.Create Foreign key with set null on delete - Using CREATE TABLE statement Syntax A simple example of PROTECT being used is If you were developing an ATM application, and you don’t want an ATM instance deleted if it has money in it.Īn important point to note is that, when you make an attempt to delete a post with comment(s), and it has an on_delete constraint of PROTECT, it will raise PROTECTEDERROR, which you can handle when creating your views. This might not look like a reasonable thing to do in our use case but it has its use cases. Put simply, Django will prevent a post from deletion if it already has comments. The PROTECT argument of the ForeignKey on_delete option prevents the referenced object from being deleted if it already has an object referencing it in the database. While using the CASCADE argument of on_delete option, it is important to think carefully about the use cases, as using this carelessly could wipe relevant data in your database you do not want to go. As expected, deleting a post also deletes all comments referencing it.