SQL Single-Row Subquery

Single-Row Subquery

Single-Row Subquery is a type of subquery that returns only one row of data to the outer query.

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1. Overview

A Single-Row Subquery is a subquery that returns exactly one row of data to the outer query. It is typically used in conditions where only a single value is expected or allowed, such as comparisons using the =, >, <, >=, <= operators, or in conjunction with aggregate functions.

Example:

// Example of a single-row subquery used in a WHERE clause
SELECT product_name
FROM products
WHERE product_id = (SELECT MAX(product_id) FROM products);

In this example, the single-row subquery retrieves the maximum product ID from the products table, which is then used as a condition to select the corresponding product name.

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2. Syntax

The syntax for a Single-Row Subquery is similar to that of a standard subquery, but it must return exactly one row:

SELECT column_name
FROM table_name
WHERE column_name operator (SELECT single_value FROM table_name);

Where single_value is the result of the subquery, which should return only one value.

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3. Usage

Single-Row Subqueries are commonly used in scenarios where a single value is required for comparison or calculation purposes. They are often used in conjunction with aggregate functions, or when retrieving specific data based on a condition that returns only one result.

Example:

// Example of using a single-row subquery to retrieve employee details based on their salary
SELECT employee_id, employee_name
FROM employees
WHERE salary > (SELECT AVG(salary) FROM employees);

This example retrieves employee IDs and names for employees whose salary is greater than the average salary of all employees.

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4. Conclusion

Single-Row Subqueries are a useful tool in SQL for retrieving specific data or performing calculations based on conditions that yield only one result. By returning a single row of data, they enable precise filtering and comparison within SQL queries.