In the case we just saw of the deletion of a course, the database can also prevent a referential integrity violation if it also deletes the sections that reference the deleted course sections 1 and 5 in the Section table.
This is called a cascade , because the deletion in the primary-key table is cascaded to the foreign-key table. Other options the database has are to set the foreign key to null or to its default value as long as the default value references an existing value in the primary-key table.
The table below summarizes all the data modifications that can take place, their impacts on referential integrity, and the choices the database has in preventing the violations in each case. Notice that the database has choices in the event of a referential integrity violation disallow, cascade, set the foreign key to null or its default value but only when an update or a delete is performed in the primary-key table; in all other cases where there would be a referential integrity violation, the only action the database can take is to disallow the operation.
That explains why only the delete and update operations are represented in the dialog you saw above. It also shows that the table in which the deletions and updates in question occur is the primary-key table. Run the script below in SQL Server or higher to create a database called SchoolEnrollment with the Course and Section tables related as shown above. After you run the script above, refresh the database folder so you can see the new database right-click on the database folder in Object Explorer and select Refresh.
Next, expand the Databases folder, then the SchoolEnrollment folder, the Tables folder, the Section table folder, and finally the Keys folder. This will put the table in design mode and will display the Foreign Key Relationships window. By default, no action is specified for either operation.
If no action is specified, the database will not allow the deletion or update in the primary-key table if they would result in referential integrity violations.
You can also select Cascade, in which case the database will allow the deletion or update in the primary-key table, but it will also cascade them to the foreign-key table as discussed above. Other choices include Set Null, which, if the delete or update will result in a referential integrity violation, the database will put Null into the CourseID foreign key on the corresponding section row s , and Set Default, which will instead use the default value of the foreign key column if one exists.
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In order to understand this, we need two database tables. For example, consider the above 2 tables Person and Gender. We will not be able to tell the Gender for this row. If we want to delete or update a record in the parent table in this case Gender table when they have a corresponding child record in the child table in this case Person table we are provided with a set of rules to perform delete and update operations known as DELETE rules and UPDATE rules as shown in the below image.
Note: The Delete rules and update rules were not imposed on the master table, they are imposed on the child table that too on the foreign key column.
Please have a look at the following diagram for the syntax. After making the relationship with cascade rules now we can perform the update and delete operations on parent table reference column data and affected the corresponding child table reference column data also.
When we execute the above query it will give us the below error. Fetch the records from the Person table and observe. This is the same as the referential integrity we have learned earlier.
Under Parent select ID in the first field. Under Child select ParentID. Your screen should look like this. Check existing data on creation — Applies the constraint to data that already exists in the database when the relationship is added to the foreign key table. Click Close and Save the table. Note that we are informed saving this table will also save the modifications that needed to be made to other tables i.
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