SQL Subqueries: Single Row Subqueries Explained
Quick Answer
Single row subqueries in SQL return exactly one row and one column. They are used in WHERE or HAVING clauses to compare a value against a single result, enabling more dynamic and precise queries.
Learning Objectives
- Understand what single row subqueries are and when to use them.
- Learn the syntax and structure of single row subqueries in SQL.
- Practice writing SQL queries using single row subqueries with comparison operators.
Introduction
SQL subqueries allow you to embed one query inside another to perform more complex data retrieval.
Single row subqueries are a type of subquery that returns exactly one row and one column, making them ideal for comparisons in WHERE clauses.
Think of single row subqueries as a precise answer to a specific question within your larger query.
What Are Single Row Subqueries?
A single row subquery returns exactly one row with one column. This makes it suitable for use with comparison operators in SQL statements.
If a subquery returns more than one row, SQL will raise an error when used in a context expecting a single value.
- Return one row and one column
- Used with operators like =, <, >, <=, >=, <>
- Commonly found in WHERE or HAVING clauses
Syntax of Single Row Subqueries
The general syntax involves placing a SELECT statement inside parentheses, which returns a single value.
This subquery is then compared to a column or value in the outer query.
- SELECT column_name FROM table WHERE column operator (subquery);
- The subquery must return only one value to avoid errors.
Examples of Single Row Subqueries
Let's look at practical examples to understand how single row subqueries work.
Example 1: Find Employees with Salary Equal to the Highest Salary
This query finds employees whose salary matches the highest salary in the company.
Example 2: Find Products Priced Below the Average Price
This query retrieves products priced less than the average price of all products.
Common Errors with Single Row Subqueries
A common mistake is using a subquery that returns multiple rows where only one is expected.
This causes SQL errors like 'subquery returns more than one row'.
- Ensure subquery returns only one row and one column.
- Use aggregate functions like MAX(), MIN(), AVG() to guarantee single value.
- Use LIMIT 1 if supported and appropriate.
Practical Example
This query uses a single row subquery to find the maximum salary and then selects employees earning that salary.
This query finds products with a price less than the average price, using a single row subquery.
Examples
SELECT employee_name, salary
FROM employees
WHERE salary = (SELECT MAX(salary) FROM employees);This query uses a single row subquery to find the maximum salary and then selects employees earning that salary.
SELECT product_name, price
FROM products
WHERE price < (SELECT AVG(price) FROM products);This query finds products with a price less than the average price, using a single row subquery.
Best Practices
- Always verify that your subquery returns a single value when using single row subqueries.
- Use aggregate functions to ensure a single result.
- Test subqueries independently to confirm their output before embedding them.
- Use meaningful aliases to improve query readability.
Common Mistakes
- Using subqueries that return multiple rows in a single row context.
- Forgetting to handle NULL values returned by subqueries.
- Not optimizing subqueries, leading to performance issues.
Hands-on Exercise
Write a Query Using Single Row Subquery
Write a SQL query to find customers whose order amount equals the maximum order amount.
Expected output: List of customers with the highest order amount.
Hint: Use MAX() in the subquery to get the highest order amount.
Identify Errors in Subqueries
Given a query with a subquery that returns multiple rows, identify and fix the error.
Expected output: Corrected query that returns a single row.
Hint: Check if the subquery needs an aggregate function or a LIMIT clause.
Interview Questions
What is a single row subquery in SQL?
InterviewA single row subquery returns exactly one row and one column and is used with comparison operators in SQL queries.
What happens if a single row subquery returns multiple rows?
InterviewSQL will raise an error because the context expects only one value, so multiple rows cause ambiguity.
How can you ensure a subquery returns a single row?
InterviewUse aggregate functions like MAX(), MIN(), or LIMIT the result to one row.
MCQ Quiz
1. What is the best first step when learning Single Row Subqueries?
A. Understand the purpose and basic idea
B. Skip directly to advanced implementation
C. Ignore examples and practice
D. Memorize terms without context
Correct answer: A
Starting with the purpose and basic idea makes later examples and practice easier to understand.
2. Which activity helps reinforce Single Row Subqueries?
A. Reading once without practice
B. Building or writing a small practical example
C. Avoiding review questions
D. Skipping the summary
Correct answer: B
A small practical example helps connect the topic to real usage.
3. Which statement is most accurate about this topic?
A. Single row subqueries in SQL return exactly one row and one column.
B. Single Row Subqueries never needs examples
C. Single Row Subqueries is unrelated to practical work
D. Single Row Subqueries should be learned without checking results
Correct answer: A
The correct option is based on the available topic explanation.
Key Takeaways
- Single row subqueries return one row and one column.
- They are commonly used with comparison operators like =, <, >, <=, >=, and <>.
- Using single row subqueries can simplify complex queries by breaking them into parts.
- Incorrect use of single row subqueries can cause errors if multiple rows are returned.
- Single row subqueries in SQL return exactly one row and one column.
Summary
Single row subqueries are a powerful SQL feature that allow you to compare a value to a single result from another query.
They simplify complex queries and help retrieve precise data when used correctly.
Always ensure your subquery returns exactly one row and one column to avoid errors.
Frequently Asked Questions
Can a single row subquery return NULL?
Yes, if the subquery finds no matching rows or the result is NULL, the subquery returns NULL.
What operators can be used with single row subqueries?
Operators like =, <, >, <=, >=, and <> are commonly used with single row subqueries.
How do single row subqueries differ from multiple row subqueries?
Single row subqueries return one row and one column, suitable for comparison operators, while multiple row subqueries return multiple rows and require operators like IN or EXISTS.
