How to Create A Foreign Key In Oracle?

To create a foreign key in Oracle, you need to first ensure that there is a primary key or unique constraint defined on the parent table. Then, you can create the foreign key constraint on the child table by using the ALTER TABLE statement with the ADD CONSTRAINT clause.

When creating the foreign key, you need to specify the columns in the child table that will reference the primary key or unique constraint in the parent table. You also need to specify the name of the foreign key constraint, as well as the name of the primary key or unique constraint that it references.

Additionally, you can define actions for the foreign key constraint using the ON DELETE and ON UPDATE clauses. These actions specify what should happen to the child rows when the parent row is deleted or updated.

Overall, creating a foreign key in Oracle involves defining the relationship between tables by referencing a primary key or unique constraint in the parent table and specifying actions for maintaining data integrity.

What is the difference between unique key and foreign key constraints in Oracle?

Unique key and foreign key constraints are both used in database tables to enforce data integrity, but they serve different purposes.

Unique key constraint:

• Ensures that all values in a specific column or set of columns are unique within a table.
• Can be used to enforce the uniqueness of a primary key or to create a unique constraint on a non-primary key column.
• Can be used to prevent duplicate values in a column or set of columns.
• Allows for null values, but each non-null value must be unique within the column(s).
• A table can have multiple unique key constraints.

Foreign key constraint:

• Creates a relationship between two tables by enforcing referential integrity.
• A foreign key in one table refers to the primary key in another table. This establishes a parent-child relationship between the two tables.
• Ensures that the values in the foreign key column(s) match the values in the primary key column(s) of the referenced table.
• Prevents actions that would violate the relationship between the two tables, such as deleting a row in the parent table if there are corresponding rows in the child table.
• Helps maintain data consistency and prevent orphaned records.
• A table can have multiple foreign key constraints that reference different tables.

In summary, unique key constraints ensure uniqueness within a single table, while foreign key constraints establish relationships between tables.

How to identify the parent table of a foreign key constraint in Oracle?

You can identify the parent table of a foreign key constraint in Oracle by looking at the definition of the foreign key constraint itself.

1. Use the following query to view all foreign key constraints in the database:

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SELECT constraint_name, table_name, r_constraint_name, delete_rule
FROM user_constraints
WHERE constraint_type = 'R';

1. Look for the foreign key constraint that you are interested in and note down the constraint_name and r_constraint_name.
2. Use the following query to find the parent table of the foreign key constraint:

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SELECT table_name
FROM user_constraints
WHERE constraint_name = 'your_constraint_name';

Replace 'your_constraint_name' with the actual name of the foreign key constraint that you are interested in. This will give you the name of the parent table of the foreign key constraint.

How to disable and enable foreign key constraints in Oracle for data loading?

To disable foreign key constraints in Oracle before data loading:

1. Open SQL*PLUS or any SQL client tool and connect to your Oracle database using an appropriate user account.
2. Run the following SQL command to disable all foreign key constraints in the database:

ALTER TABLE table_name DISABLE CONSTRAINT constraint_name;

Replace "table_name" with the name of the table containing the foreign key constraint and "constraint_name" with the name of the foreign key constraint.

1. Repeat the above command for each foreign key constraint that you want to disable.

To enable foreign key constraints after data loading:

1. Once the data loading process is complete, run the following SQL command to enable all foreign key constraints in the database:

ALTER TABLE table_name ENABLE CONSTRAINT constraint_name;

Replace "table_name" with the name of the table containing the foreign key constraint and "constraint_name" with the name of the foreign key constraint.

1. Repeat the above command for each foreign key constraint that you want to enable.

Remember to disable foreign key constraints before loading data and enable them again after loading to ensure data integrity in your Oracle database.