UNIQUE Key FAQS

 1. What is a UNIQUE constraint in Oracle?

A UNIQUE constraint ensures that all values in a specified column (or combination of columns) are distinct across all rows in the table. It prevents duplicate non-NULL values, but multiple NULL values are allowed.

2. Can a UNIQUE constraint allow NULL values?

Yes, a UNIQUE constraint allows multiple NULL values. In Oracle, NULL is considered a special marker and is not treated as a duplicate of another NULL value. Therefore, you can have multiple rows with NULL in a column with a UNIQUE constraint.

3. How do I define a UNIQUE constraint in Oracle?

You can define the UNIQUE constraint during table creation or add it to an existing table.

• During table creation:

CREATE TABLE employees (

    employee_id NUMBER PRIMARY KEY,

    email VARCHAR2(100) UNIQUE

);

• Adding to an existing table:

ALTER TABLE employees

ADD CONSTRAINT unique_email UNIQUE (email);

4. What is the difference between UNIQUE and PRIMARY KEY constraints?

• PRIMARY KEY: Ensures that values are both unique and not NULL. A table can have only one PRIMARY KEY constraint.
• UNIQUE: Ensures values are unique but allows NULL values. A table can have multiple UNIQUE constraints.

5. Can I have multiple UNIQUE constraints in one table?

Yes, a table can have multiple UNIQUE constraints. You can apply UNIQUE to different columns or combinations of columns, as long as they ensure uniqueness.

6. Can a column with a UNIQUE constraint have duplicate values?

No, a column with a UNIQUE constraint cannot have duplicate non-NULL values. If you try to insert a duplicate value into that column, Oracle will raise a constraint violation error.

7. How does Oracle handle NULL values in a UNIQUE column?

Oracle allows multiple NULL values in a column with a UNIQUE constraint. Since NULL is not considered equal to another NULL, multiple rows with NULL in the UNIQUE column are allowed.

8. Can I create a UNIQUE constraint on multiple columns (composite key)?

Yes, you can create a UNIQUE constraint on a combination of columns, ensuring that the combination of values across the specified columns is unique.

Example:

CREATE TABLE orders (

    order_id NUMBER PRIMARY KEY,

    customer_id NUMBER,

    product_id NUMBER,

    CONSTRAINT unique_customer_product UNIQUE (customer_id, product_id)

);

9. What happens if I insert a duplicate value in a UNIQUE column?

If you attempt to insert a duplicate value (that is not NULL) into a column with a UNIQUE constraint, Oracle will raise an error, and the insertion will fail.

10. How does a UNIQUE constraint affect performance?

• Insert Performance: The UNIQUE constraint can slightly affect insert performance because Oracle must check for duplicates before inserting a row.
• Indexing: Oracle automatically creates a unique index on the column(s) defined with a UNIQUE constraint. This helps with query performance but adds overhead on INSERT, UPDATE, and DELETE operations.

11. How can I drop a UNIQUE constraint in Oracle?

To drop a UNIQUE constraint, you can use the ALTER TABLE statement:

ALTER TABLE employees

DROP CONSTRAINT unique_email;

12. Can I have both UNIQUE and PRIMARY KEY constraints on the same column?

No, a column can only have one PRIMARY KEY constraint. However, a column with a PRIMARY KEY automatically enforces uniqueness, so it does not need a separate UNIQUE constraint.

13. Can I create a UNIQUE constraint on a column with existing duplicate data?

No, you cannot apply a UNIQUE constraint to a column that already contains duplicate non-NULL values. You must first remove or update the duplicate rows before applying the constraint.

14. What happens if I try to insert a NULL value in a column with a UNIQUE constraint?

You can insert NULL values in a column with a UNIQUE constraint. Since NULL is not considered a duplicate of another NULL, multiple rows with NULL in the UNIQUE column are allowed.

15. Can a UNIQUE constraint be created without an index?

No, when you define a UNIQUE constraint, Oracle automatically creates a unique index on the column(s). This index ensures that the uniqueness rule is enforced efficiently.

16. Can I modify a UNIQUE constraint after it is applied?

Yes, you can modify a UNIQUE constraint, but you might need to drop and recreate it if you want to change the columns involved. Use the ALTER TABLE statement to modify constraints.

17. What is a UNIQUE constraint used for?

The UNIQUE constraint is used to ensure that a column (or combination of columns) does not have duplicate values, which is essential for maintaining data integrity in a database. It's commonly used for enforcing the uniqueness of fields such as email addresses, usernames, or product codes.