A subquery in SQL is essentially a query nested within another query. It can retrieve data that will be used in the main query as a condition to further refine the data that is to be retrieved. Subqueries are powerful tools, allowing complex database operations to be broken down into simpler ones.
Subqueries can be used in various SQL operations, including:
These subqueries can be used to return a single value or a list of values depending on the need. They are commonly placed in the SELECT clause of the SQL statement.
SELECT column_name,
(subquery returning a single value) AS column_alias_name
FROM table_name;
Example:
SELECT EmployeeName,
(SELECT COUNT(*) FROM Orders WHERE EmployeeID = E.EmployeeID) AS NumberOfOrders
FROM Employees E;
Subqueries can also be used in the WHERE clause to filter out records from the main query. These subqueries are used to return a single value or a list of values to be used by the main query.
SELECT column_name
FROM table_name
WHERE column_name operator (subquery returning a list of values);
Example:
SELECT EmployeeName
FROM Employees
WHERE EmployeeID IN (SELECT EmployeeID FROM Orders WHERE OrderDate = '2023-08-30');
SELECT E.EmployeeName, COUNT(O.OrderID) AS NumberOfOrders
FROM Employees E
LEFT JOIN Orders O ON E.EmployeeID = O.EmployeeID
GROUP BY E.EmployeeName
HAVING COUNT(O.OrderID) > 10;
The `JOIN` approach is often more efficient and readable than the correlated subquery approach for tasks like aggregating data based on relationships.
Subqueries offer a way to perform multiple data retrievals in a single query, breaking down complex operations into more manageable parts. They bring flexibility and power to SQL operations, enabling intricate database tasks with greater efficiency and clarity.