Sub-Queries and Nested Selects

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 Introduction to Subqueries:


Subqueries are regular queries nested inside another query, enclosed within parentheses.

They allow for more complex and powerful queries.

Scenario: Retrieving Employees with Salaries Above Average:


Attempting to directly compare salaries with the average using aggregate functions in the WHERE clause results in errors.

To overcome this limitation, subqueries can be used to calculate the average salary separately and then compare it with individual salaries.

Example: SELECT EmployeeID, FirstName, LastName, Salary FROM Employees WHERE Salary > (SELECT AVG(Salary) FROM Employees);

Column Expressions:


Subqueries can also be used within the list of columns to be selected.

Example: SELECT EmployeeID, Salary, (SELECT AVG(Salary) FROM Employees) AS AverageSalary FROM Employees;

Derived Tables or Table Expressions:


Subqueries can be used in the FROM clause to create derived tables or table expressions.

These derived tables provide a way to manipulate and filter data before presenting it to the outer query.

Example: SELECT * FROM (SELECT EmployeeID, FirstName, LastName, DepartmentID FROM Employees) AS EmployeeInfo;

Benefits of Subqueries:


Subqueries allow for more flexible and expressive queries, overcoming limitations of direct aggregate function usage in certain contexts.

They enable the creation of derived tables to handle complex data manipulation and filtering tasks.

By understanding and leveraging subqueries in various parts of your SQL queries, you can write more efficient and powerful database queries to meet your data retrieval and manipulation needs.





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