Single-row Subqueries in SQL

A single-row subquery is a subquery that returns exactly one row of data, which can be used in a WHERE or HAVING clause. It is commonly used when you need to compare a value from the outer query to the result of the subquery.

1. What is a Single-row Subquery?

A single-row subquery is a subquery that returns only one row of data and can be used with comparison operators like =, !=, >, <, etc. These subqueries can return one or more columns, but only one row of data should be returned. If the subquery returns multiple rows, an error will occur.

Single-row subqueries are typically used in situations where a column in the outer query needs to be compared to a single value or a derived value from another table.

2. Syntax of Single-row Subquery

The syntax of a single-row subquery is as follows:

      SELECT column1, column2, ...
      FROM table1
      WHERE column1 = (SELECT column1 FROM table2 WHERE condition);
    

In this syntax:

• table1: The outer table from which data is selected.
• column1, column2: The columns that are retrieved from table1.
• table2: The inner table from which data is selected for comparison.
• The subquery should return exactly one row of data for the comparison to work.

3. Example of Single-row Subquery

Suppose you have two tables: Employees and Departments. You want to find employees who work in the department that has the highest salary budget.

      SELECT EmployeeName
      FROM Employees
      WHERE DepartmentID = (SELECT DepartmentID FROM Departments WHERE Budget = (SELECT MAX(Budget) FROM Departments));
    

This query works as follows:

• The subquery (SELECT MAX(Budget) FROM Departments) returns the highest budget value.
• The inner subquery (SELECT DepartmentID FROM Departments WHERE Budget = ...) uses this highest budget value to find the department ID with the highest budget.
• The outer query selects the employee names from the Employees table where the DepartmentID matches the department ID returned by the subquery.

4. Example of Single-row Subquery with Comparison

Let's say you have a Products table with columns ProductID, ProductName, and Price. You want to find products that cost more than the average price in the Products table.

      SELECT ProductName, Price
      FROM Products
      WHERE Price > (SELECT AVG(Price) FROM Products);
    

In this query:

• The subquery (SELECT AVG(Price) FROM Products) calculates the average price of all products.
• The outer query retrieves products whose price is greater than this average.

5. Key Points About Single-row Subqueries

• A single-row subquery returns exactly one row. If it returns more than one row, an error will occur.
• Single-row subqueries can be used with comparison operators like =, >, <, >=, <=, !=.
• If the subquery does not return any rows, the result of the outer query will be an empty result set.

6. Using Single-row Subqueries with Other SQL Clauses

Single-row subqueries can also be used with other SQL clauses such as HAVING or even in the SELECT list. Here are a few examples:

Using a Single-row Subquery in HAVING

      SELECT DepartmentID, COUNT(*) AS NumberOfEmployees
      FROM Employees
      GROUP BY DepartmentID
      HAVING COUNT(*) > (SELECT AVG(NumberOfEmployees) FROM (SELECT DepartmentID, COUNT(*) AS NumberOfEmployees FROM Employees GROUP BY DepartmentID) AS DeptCount);
    

In this example:

• The inner subquery (SELECT AVG(NumberOfEmployees) ...) calculates the average number of employees per department.
• The outer query groups employees by department and returns only those departments with more employees than the average.

Using a Single-row Subquery in SELECT

      SELECT EmployeeName, (SELECT DepartmentName FROM Departments WHERE DepartmentID = Employees.DepartmentID) AS Department
      FROM Employees;
    

In this query, the subquery fetches the department name for each employee based on their DepartmentID.

7. Conclusion

Single-row subqueries are a powerful way to return a single value or derived value from another query, which can then be used in the outer query's WHERE, HAVING, or SELECT clauses. They are useful for comparisons and can simplify queries by eliminating the need for joins in some cases.