Scalar Subqueries FAQS

 1. What is a Scalar Subquery in Oracle?

A Scalar Subquery is a subquery that returns exactly one value (one row and one column). This value can be a constant, an aggregate result (like MAX(), AVG(), SUM(), etc.), or a computed expression. Scalar subqueries are used where a single value is required, such as in the SELECT, WHERE, or UPDATE clauses.

2. Where can I use Scalar Subqueries in SQL?

Scalar subqueries can be used in:

• SELECT clause: To calculate a single value for each row.
• WHERE clause: To filter results based on a single value returned by the subquery.
• SET clause of UPDATE: To assign a value based on a subquery result.
• VALUES clause of INSERT: To insert values calculated by a subquery.

3. What happens if a Scalar Subquery returns more than one row?

If a scalar subquery returns more than one row, Oracle will throw the following error:

ORA-01427: single-row subquery returns more than one row

To avoid this, ensure that your subquery only returns a single row.

4. Can a Scalar Subquery return multiple columns?

No, a scalar subquery must return exactly one column. If the subquery returns multiple columns, it will result in an error. You can, however, use aggregate functions (e.g., MAX(), AVG()) to summarize multiple rows into one value.

5. What happens if a Scalar Subquery returns no rows?

If a scalar subquery returns no rows, it will return NULL. If the subquery is used in a comparison (e.g., WHERE salary > (subquery)), the result will be treated as FALSE due to the NULL value.

6. How do I handle NULL values in Scalar Subqueries?

You can handle NULL values in scalar subqueries using functions like NVL() or COALESCE() to replace NULL with a default value. For example:

SELECT employee_id, NVL((SELECT MAX(salary) FROM employees WHERE department_id = 10), 0) AS max_salary

FROM employees;

This will replace any NULL result with 0.

7. How can I optimize Scalar Subqueries for performance?

• Avoid repeating subqueries: If the scalar subquery is used multiple times, consider joining the data instead of repeating the subquery.
• Use indexes: Make sure the columns used in the subquery (like department_id, salary, etc.) are indexed to improve performance.
• Check for large result sets: Although scalar subqueries return only one row, if the inner query is slow, it can still affect overall performance.

8. Can Scalar Subqueries be used in INSERT statements?

Yes, scalar subqueries can be used in the VALUES clause of an INSERT statement to insert values based on the result of a subquery. For example:

INSERT INTO employees (employee_id, name, salary)

VALUES (1001, 'John Doe', (SELECT MAX(salary) FROM employees) + 5000);

This will insert a new employee with a salary 5,000 more than the highest salary in the employees table.

9. What is the difference between Scalar Subqueries and Multi-Row Subqueries?

• Scalar Subquery: Returns a single value (one row and one column). It’s used where exactly one value is needed (e.g., in a comparison).
• Multi-Row Subquery: Returns multiple rows (but only one column). It’s used with operators like IN, ANY, or ALL to compare against multiple values.

10. How can I ensure a Scalar Subquery returns only one row?

To ensure a scalar subquery returns a single row, you can:

• Use an aggregate function like MAX(), MIN(), AVG(), etc., which will always return a single value.
• Use the ROWNUM or FETCH FIRST clause to limit the result to one row.

Example:

SELECT employee_id, (SELECT MAX(salary) FROM employees WHERE department_id = 10) AS max_salary

FROM employees;

This guarantees that only one row is returned, even if there are multiple employees in department 10.

11. Can Scalar Subqueries be used with Aggregate Functions?

Yes, scalar subqueries are commonly used with aggregate functions to compute values like sums, averages, or maximum values. For example:

SELECT employee_id, salary

FROM employees

WHERE salary > (SELECT AVG(salary) FROM employees);

This query finds employees who earn more than the average salary.

12. Can I use Scalar Subqueries in the UPDATE statement?

Yes, scalar subqueries are often used in the SET clause of an UPDATE statement to assign a value derived from another table. For example:

UPDATE employees

SET salary = salary + (SELECT AVG(salary) FROM employees WHERE department_id = 10)

WHERE department_id = 10;

This query increases the salary of all employees in department 10 by the average salary in that department.

13. What is the best approach to avoid errors when using Scalar Subqueries?

To avoid errors like ORA-01427 (subquery returns more than one row), ensure that:

• Your subquery always returns one row.
• Use aggregate functions to summarize multiple rows into one value.
• If needed, use the ROWNUM or FETCH FIRST clause to limit the subquery result to a single row.

14. Can Scalar Subqueries be nested?

Yes, scalar subqueries can be nested inside another scalar subquery, as long as the final result is a single value. For example:

SELECT employee_id, (SELECT MAX(salary) FROM employees WHERE department_id = (SELECT department_id FROM departments WHERE location_id = 1000)) AS max_salary

FROM employees;

This nested query finds the highest salary in the department located in location 1000.