DEFAULT Constraint FAQS

1. What is the purpose of the DEFAULT constraint in Oracle?

The DEFAULT constraint automatically assigns a default value to a column when an explicit value is not provided during an INSERT operation. This helps ensure that columns always have a value, preventing errors or the need for additional logic in application code.

 

2. Can I specify the DEFAULT value as an expression or function?

Yes, you can use expressions or functions as default values. For example:

• DEFAULT SYSDATE: The current system date.
• DEFAULT USER: The current database user.
• DEFAULT (salary * 1.1): You can also use expressions, although such defaults are better handled with triggers in Oracle.

 

3. What happens if I insert a record without specifying a value for a column with a DEFAULT constraint?

If a column has a DEFAULT constraint and the value for that column is not provided in the INSERT statement, Oracle will automatically insert the default value specified in the table definition.

Example:

INSERT INTO employees (employee_id, first_name, last_name)

VALUES (1, 'John', 'Doe');

If hire_date has a default of SYSDATE, then the current system date will be inserted for the hire_date column.

 

4. What happens if I insert NULL into a column with a DEFAULT constraint?

If a column allows NULL values and you explicitly insert NULL for a column with a DEFAULT constraint, Oracle will insert NULL rather than the default value.

Example:

INSERT INTO employees (employee_id, first_name, last_name, salary)

VALUES (2, 'Jane', 'Smith', NULL);

Here, if salary has a default value of 50000, it will not be used, and NULL will be inserted instead.

 

5. Can I change the default value of an existing column in Oracle?

Yes, you can change the default value of a column using the ALTER TABLE statement.

Example:

ALTER TABLE employees

MODIFY salary DEFAULT 55000;

This will change the default value for the salary column to 55000.

 

6. Can I remove the DEFAULT constraint from a column?

Yes, you can remove the DEFAULT constraint by modifying the column or using the ALTER TABLE statement.

Example:

ALTER TABLE employees

MODIFY salary DEFAULT NULL;

This removes the default value for the salary column.

 

7. Is it possible to have multiple default values for different rows in the same column?

No, a column can have only one default value at a time. However, you can have different default values for different rows by using conditional logic within a TRIGGER, but not directly via the DEFAULT constraint.

 

8. Can a DEFAULT constraint be used on columns that allow NULL values?

Yes, the DEFAULT constraint can be used on columns that allow NULL values. If no value is provided, Oracle will insert the default value. However, if a NULL value is explicitly provided, the default value will not be applied, and NULL will be inserted.

 

9. Can I use DEFAULT with a column that is part of a PRIMARY KEY or UNIQUE constraint?

Yes, you can use the DEFAULT constraint with columns that are part of a PRIMARY KEY or UNIQUE constraint. However, the default value must not conflict with the uniqueness of the constraint. For example, if the default value is 10, it should not conflict with other PRIMARY KEY values.

 

10. What happens if I insert a record with a value that violates the column's constraints (e.g., NOT NULL) but the column has a DEFAULT value?

If a column has a DEFAULT value and also has a NOT NULL constraint, the DEFAULT value will be inserted if no value is provided, and the record will be valid. However, if you explicitly insert a NULL value into a column that is NOT NULL, an error will be raised.

Example:

CREATE TABLE employees (

    employee_id NUMBER PRIMARY KEY,

    first_name VARCHAR2(50),

    last_name VARCHAR2(50),

    salary NUMBER DEFAULT 50000 NOT NULL

);

In this case, trying to insert NULL for salary would result in an error.

 

11. Can I use DEFAULT with CHECK constraints?

Yes, you can use DEFAULT in combination with CHECK constraints, but the default value must meet the criteria defined in the CHECK constraint.

Example:

CREATE TABLE employees (

    employee_id NUMBER PRIMARY KEY,

    salary NUMBER DEFAULT 50000 CHECK (salary > 0)

);

Here, the default salary is 50000, and the CHECK constraint ensures it is greater than 0.

 

12. Are there any limitations with the DEFAULT constraint in Oracle?

• No complex conditions: You cannot have complex conditions or multiple values based on other columns directly in the DEFAULT constraint.
• Cannot use DEFAULT for auto-increment columns: For auto-incrementing columns, Oracle uses sequences or identity columns, not DEFAULT constraints.

 

13. Can I specify a DEFAULT value for a column in a VIEW?

No, the DEFAULT constraint is only applicable to columns in tables. You cannot directly define a DEFAULT value for columns in a VIEW.

 

14. Can I apply DEFAULT to a column that has a FOREIGN KEY constraint?

Yes, a DEFAULT constraint can be applied to a column with a FOREIGN KEY constraint. However, the default value must exist in the referenced table, or the FOREIGN KEY constraint will be violated.

Example:

CREATE TABLE departments (

    department_id NUMBER PRIMARY KEY,

    department_name VARCHAR2(100)

);

 

CREATE TABLE employees (

    employee_id NUMBER PRIMARY KEY,

    department_id NUMBER DEFAULT 10,

    FOREIGN KEY (department_id) REFERENCES departments(department_id)

);

Here, if department_id has a default of 10, that value must exist in the departments table for the FOREIGN KEY constraint to be valid.

 

15. How does the DEFAULT constraint behave with INSERT INTO ... SELECT statements?

When using INSERT INTO ... SELECT, the default value will only be applied to columns that are not explicitly listed in the SELECT clause. If a column is omitted from the SELECT statement, Oracle will insert the default value for that column.

Example:

INSERT INTO employees (employee_id, first_name, last_name)

SELECT employee_id, first_name, last_name

FROM temp_employees;

In this case, the salary and hire_date columns will receive their default values if not explicitly provided.