Single-Row Subqueries FAQS

 1. What is a Single-Row Subquery?

A Single-Row Subquery is a subquery that returns at most one row and one column of results. It's often used in places where a single value is needed, such as in comparison operations (=, <, >, etc.).

2. What happens if a Single-Row Subquery returns more than one row?

If a single-row subquery returns more than one row, Oracle will throw an error:

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

3. Can I use a Single-Row Subquery with the IN operator?

No, the IN operator is used with subqueries that can return multiple rows. If you use a single-row subquery, it should be compared using operators like =, >, <, etc., not IN.

4. What are the common operators used with Single-Row Subqueries?

Single-row subqueries are typically used with comparison operators, such as:

• =, !=, <> (Equality/Not Equal)
• <, >, <=, >= (Comparison)
• BETWEEN (Range comparison)
• LIKE (Pattern matching)
• IS NULL (Checking for null values)

5. Can a Single-Row Subquery be used in the SELECT clause?

Yes, a single-row subquery can be used in the SELECT clause to return a scalar value that is the same for all rows in the outer query. For example:

SELECT employee_id, name, (SELECT MAX(salary) FROM employees) AS max_salary

FROM employees;

6. Can I use a Single-Row Subquery in the WHERE clause?

Yes, single-row subqueries are commonly used in the WHERE clause for comparisons. For example:

SELECT employee_id, name

FROM employees

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

This filters employees earning more than the average salary.

7. What is an example of an invalid Single-Row Subquery?

A common mistake is using a subquery that returns multiple rows when only one row is expected:

SELECT employee_id, name

FROM employees

WHERE department_id = (SELECT department_id FROM departments);

This will raise an error if the departments table contains more than one row.

8. How do I handle multiple rows in a subquery when using a Single-Row Subquery?

If you expect a subquery to return multiple rows, you should:

• Use IN instead of = to handle multiple values.
• Use EXISTS if you only care about the existence of rows, not the actual data.

Example using IN:

SELECT employee_id, name

FROM employees

WHERE department_id IN (SELECT department_id FROM departments);

9. What are some examples of using Single-Row Subqueries?

• Using a Single-Row Subquery in WHERE:

·        SELECT employee_id, name

·        FROM employees

·        WHERE salary > (SELECT MAX(salary) FROM employees);

• Using a Single-Row Subquery in SELECT:

·        SELECT employee_id, name, (SELECT MIN(salary) FROM employees WHERE department_id = 10) AS min_salary

·        FROM employees;

10. How do I optimize a Single-Row Subquery?

To optimize performance:

• Use indexes on columns that are referenced in the subquery.
• Ensure that the subquery only returns a single value by applying aggregate functions (MAX, MIN, AVG, etc.) if necessary.
• Avoid unnecessary subqueries and restructure the query to reduce complexity if possible.

11. Can I use Single-Row Subqueries in an UPDATE statement?

Yes, you can use single-row subqueries in an UPDATE statement. For example:

UPDATE employees

SET salary = (SELECT MAX(salary) FROM employees WHERE department_id = 10)

WHERE department_id = 20;

12. Can a Single-Row Subquery be nested inside another subquery?

Yes, single-row subqueries can be nested within other subqueries. Each level should ensure that it returns a single value.

Example:

SELECT employee_id, name

FROM employees

WHERE salary > (SELECT MAX(salary) FROM employees WHERE department_id = (SELECT department_id FROM departments WHERE department_name = 'HR'));

13. Can I use aggregate functions in a Single-Row Subquery?

Yes, aggregate functions like MAX, MIN, AVG, COUNT, and SUM are commonly used in single-row subqueries to return a single value (the result of the aggregate operation).

Example:

SELECT employee_id, name

FROM employees

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

14. Can a Single-Row Subquery return NULL?

Yes, a single-row subquery can return NULL if there are no rows or if the computation results in NULL. In such cases, the comparison with NULL will return FALSE unless you explicitly use IS NULL.

15. How do I ensure a Single-Row Subquery returns only one row?

To ensure that a subquery returns only one row, you can:

• Use aggregate functions like MAX(), MIN(), AVG(), etc., which naturally reduce the result to one value.
• Add conditions that restrict the subquery to return only a single row (e.g., using ROWNUM or FETCH FIRST).