To assign a foreign key in Oracle 10g, you need to follow the syntax:
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ALTER TABLE child_table ADD CONSTRAINT fk_constraint_name FOREIGN KEY (child_column) REFERENCES parent_table (parent_column); |
Here, child_table is the table that will contain the foreign key, while parent_table is the table that contains the primary key that the foreign key references. child_column is the column in the child table which will be the foreign key, and parent_column is the corresponding column in the parent table that is the primary key.
For example, let's assume we have two tables, employees and departments, with the following structures:
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CREATE TABLE departments ( department_id NUMBER PRIMARY KEY, department_name VARCHAR2(100) ); CREATE TABLE employees ( employee_id NUMBER PRIMARY KEY, employee_name VARCHAR2(100), department_id NUMBER, CONSTRAINT fk_department_id FOREIGN KEY (department_id) REFERENCES departments (department_id) ); |
In this case, we are assigning a foreign key constraint named fk_department_id on the employees table to link the department_id column in employees to the department_id column in departments.
Once this foreign key constraint is in place, it ensures referential integrity, meaning that any value in the department_id column of the employees table must be a valid value from the department_id column of the departments table. If an attempt is made to insert a value that doesn't exist in the referenced table, or to delete/update a referenced value, it will result in an error.
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What is the impact of dropping a primary key on foreign key constraints in Oracle?
When a primary key is dropped in Oracle, the foreign key constraints associated with that primary key are also impacted. Here are the effects:
- Disabled Constraints: Dropping a primary key will cause any foreign key constraints that reference the dropped primary key to be automatically disabled. This means that the foreign key constraints will no longer enforce referential integrity until they are re-enabled.
- Data Integrity: With the primary key dropped, the foreign key constraints are no longer able to ensure data integrity between the related tables. This may lead to the existence of orphan records, where foreign key values no longer have corresponding primary key values.
- Invalidation of Existing Data: The foreign key values that were referencing the now-dropped primary key will become invalid. This can cause issues when trying to query or update the related data.
- Inability to Re-Enable Constraints: If the primary key is dropped, and foreign key constraints are disabled as a result, re-enabling the constraints might be problematic. This is because the foreign key values may not match any existing primary key values, leading to errors.
Overall, dropping a primary key can have a significant impact on foreign key constraints, compromising data integrity and causing issues with existing data and constraint management. Therefore, it is essential to consider the ramifications and reconfigure or restore the constraint relationships appropriately after dropping a primary key.
What is the maximum number of foreign keys allowed on a table in Oracle?
There is no specific maximum number of foreign keys allowed on a table in Oracle. The number of foreign keys that can be defined on a table is generally limited by the maximum number of columns per table, which is 1000.
What happens when a foreign key references a non-existent primary key in Oracle?
When a foreign key references a non-existent primary key in Oracle, it results in a referential integrity constraint violation. This means that Oracle will not allow the insertion or update of a row that contains a foreign key value that does not exist in the referenced primary key.
If an attempt is made to insert or update a row with a foreign key value that does not exist, Oracle will throw a "ORA-02291: integrity constraint" error. This error indicates that the foreign key constraint has been violated and the operation cannot be completed.
How to find all tables referenced by foreign keys in Oracle 10g?
In Oracle 10g, you can use the following query to find all tables that are referenced by foreign keys:
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SELECT DISTINCT c.table_name FROM all_constraints c JOIN all_constraints r ON c.constraint_name = r.r_constraint_name WHERE c.constraint_type = 'R' AND r.constraint_type IN ('P', 'U'); |
This query joins the all_constraints view with itself to find all foreign key constraints (c.constraint_type = 'R') that reference primary key or unique constraints (r.constraint_type IN ('P', 'U')).
Make sure you have the necessary privileges to query the all_constraints view.
What is a foreign key in Oracle?
In Oracle, a foreign key is a column or a set of columns in a table that refers to the primary key or a unique key in another table. It establishes a relationship between two tables, known as a parent-child relationship. The foreign key constraint ensures data integrity by enforcing referential integrity rules.
What is the significance of referential integrity in relation to foreign keys?
Referential integrity is a fundamental concept in database management systems, particularly when dealing with foreign keys. It ensures that relationships between tables are maintained consistently and accurately.
In relation to foreign keys, referential integrity ensures that the values stored in a foreign key column of one table correspond to the primary key values in the referenced table. It maintains the integrity of the relationships between tables by enforcing certain rules:
- Maintaining Consistency: Referential integrity ensures that the values stored in the foreign key column always exist in the primary key column of the referenced table, preventing any orphaned or dangling records.
- Data Integrity: It helps maintain the integrity of the data by preventing the creation of "orphan" records, where a foreign key points to a non-existent primary key. It ensures that only valid and existing data is referenced.
- Data Accuracy: Referential integrity provides accuracy in data by enforcing constraints that ensure the integrity of relationships. It prevents referential data inconsistencies and helps avoid potential data corruption issues.
- Data Validation: It acts as a validation mechanism, as it checks the validity of foreign key values at the time of insertion or modification against the primary key values in the referenced table. It permits only valid data to be inserted into the foreign key column.
In summary, referential integrity ensures that relationships between tables remain consistent, accurate, and valid by enforcing constraints on foreign keys. It plays a crucial role in maintaining data integrity and preventing data inconsistencies or corruption in a database.
