Constraints Best Practices FAQS

1. What are Oracle constraints?

Constraints are rules applied to table columns to enforce data integrity and business rules.

Common types:

• NOT NULL – Column must have a value
• UNIQUE – Ensures no duplicate values
• PRIMARY KEY – Unique identifier for table rows
• FOREIGN KEY – Ensures referential integrity with another table
• CHECK – Enforces a logical condition
• DEFAULT – Assigns default values

2. Why follow best practices with constraints?

• Ensure data accuracy and consistency
• Avoid performance degradation
• Make maintenance easier
• Prevent data anomalies in multi-user systems

3. General best practices

1.    Define constraints at the correct level

o   Column-level for single-column rules

o   Table-level for multi-column rules

2.    Use constraints for business-critical rules instead of relying only on application logic

3.    Name constraints explicitly

ALTER TABLE employees ADD CONSTRAINT emp_pk PRIMARY KEY(emp_id);

4.    Use NOT NULL for mandatory fields instead of CHECK for simple existence

5.    Keep CHECK conditions simple

4. Primary Key best practices

• Every table should have a primary key
• Prefer numeric or surrogate keys (sequences)
• Avoid composite primary keys unless necessary

Example:

emp_id NUMBER PRIMARY KEY

5. Foreign Key best practices

• Enforce referential integrity
• Index foreign key columns
• Use ON DELETE CASCADE or SET NULL cautiously
• Avoid unnecessary cascading deletes

Example:

ALTER TABLE orders

ADD CONSTRAINT fk_customer

FOREIGN KEY(customer_id)

REFERENCES customers(customer_id);

6. Unique constraint best practices

• Use UNIQUE for columns requiring distinct values
• Combine with NOT NULL when needed
• Oracle automatically creates an index

7. Check constraint best practices

• Keep conditions simple and deterministic
• Avoid complex functions or subqueries
• Use for business rules like age limits or status values

Example:

ALTER TABLE employees

ADD CONSTRAINT chk_salary CHECK(salary > 0);

8. Default constraint best practices

• Use DEFAULT for optional columns
• Combine with NOT NULL if required
• Avoid sequences directly as defaults; use triggers

9. Performance considerations

• PRIMARY KEY and UNIQUE create indexes automatically
• Index foreign key columns for better performance
• Keep CHECK constraints simple on high-volume tables

10. Constraint validation strategies

• ENABLE VALIDATE – Checks existing and new data
• ENABLE NOVALIDATE – Enforces new data only
• DISABLE – Turns off enforcement temporarily

Use ENABLE NOVALIDATE during bulk loads when appropriate.

11. Handling constraints during data migration

Disable non-critical constraints:

ALTER TABLE employees DISABLE CONSTRAINT chk_salary;

Re-enable after load:

ALTER TABLE employees ENABLE VALIDATE CONSTRAINT chk_salary;

12. Naming conventions

Follow consistent naming:

Constraint Type	Example Naming
Primary Key	table_name_pk
Foreign Key	table_name_col_fk
Unique	table_name_col_uk
Check	table_name_col_chk
Default	table_name_col_def

Helps in maintenance and debugging.

13. Common mistakes to avoid

• Not using constraints for critical rules
• Overcomplicating CHECK conditions
• Not indexing foreign key columns
• Disabling constraints without re-validating
• Using sequences directly as DEFAULT

14. Best practices summary

1.    Always define primary keys

2.    Use foreign keys for referential integrity

3.    Apply UNIQUE and NOT NULL appropriately

4.    Keep CHECK constraints simple

5.    Use DEFAULT for optional columns

6.    Name constraints consistently

7.    Index key columns for performance

8.    Disable/enable constraints carefully during migration

9.    Avoid unnecessary constraints on high-volume tables

10.                    Document constraints for maintainability

15. Interview Tip

“Use constraints to enforce data integrity at the database level. Always define primary keys, use foreign keys with indexed columns, apply unique and not null constraints appropriately, keep check constraints simple, use default values for optional columns, follow consistent naming conventions, and validate constraints carefully during bulk data operations. Proper constraint design improves data quality and reduces application-side validation.”