SQL - Derived Tables

Derived tables is the most outstanding feature of SQL which provides a momentous help to the developers because they are always created on the fly with the help of SELECT and FROM statements in parenthesis and can also be used in queries such as Sub-Queries, Common Table Expressions, and Views. The amazing point is that no need to drop it because SQL Server is the responsible to create and populate the table in the memory which can be referenced by the outer Select query who created.

To simplify it, a derived table is best thought of as a table within a table that exists only as long as the outer query does. They can assumed as on the fly views also in SQL.

Drawback Points: A derived table only exists in the query in which it is created. So, derived tables are not actually part of the database schema because they are not real tables. SQL server will eliminate it forever from the memory when the outer query is finished. Reusable Options: If you want to re-use the output of the Derived table in the SQL query then you must need to be store its output into the table variable or temporary table because they are available till the end  of the query session.

Due to reside in the memory, it would be faster the temporary tables which are created in the temp database but in case of if you are fetching few records from the database. SQL server will eliminate it when the outer query is finished.

Explanation of Derived tables: Use of Derived tables are depend on the requirements and we are explaining some of them here. An employee table is used to store all the necessary information of the employees and department master is used to store department related details

USE TEMPDB GO SET NOCOUNT ON ---- Create table employee CREATE TABLE Employees ( EmpId Int NOT NULL, EmpName Varchar(20) NOT Null, EmpSalary money not null, DeptId Int Not Null, EmpAge Int Not Null ) GO ----- Insert values into table INSERT INTO Employees (EmpId, EmpName, EmpSalary, DeptId,EmpAge) VALUES (101, 'Ryan Arjun', 50000, 51, 23), (102, 'Kimmy Wang', 42000, 52, 29), (103, 'Bill Smith', 45000, 52, 35), (104, 'Rose White', 25000, 53, 21), (105, 'Mac Johnson', 30000, 51, 32)   GO ----- Output from the table select * from Employees; EmpId EmpName EmpSalary DeptId EmpAge 101 Ryan Arjun 50000 51 23 102 Kimmy Wang 42000 52 29 103 Bill Smith 45000 52 35 104 Rose White 25000 53 21 105 Mac Johnson 30000 51 32 ----- create another table department CREATE TABLE Department ( DeptID INT Not Null, DeptName Varchar(20) Not Null ) GO ---- Insert Values into table INSERT Department VALUES (51, 'Accounts'), (52, 'Sales'), (53, 'Marketing'), (54, 'Admin') GO ----- pull values from department table select * from Department; DeptID DeptName 51 Accounts 52 Sales 53 Marketing 54 Admin ---- pull values select e1.EmpId, e1.EmpName, e1.EmpSalary, dpt.DeptName from Department dpt ----- derived table inner join ( select EmpId, EmpName, DeptId, EmpSalary from Employees where EmpSalary>42000 and EmpName like 'R%' )E1 on dpt.DeptID=E1.DeptId EmpId EmpName EmpSalary DeptName 101 Ryan Arjun 50000 Accounts ---- DROP TABLES DROP TABLE Employees, Department

Referencing Multiple Derived Tables- As we are working with the views, the same way, you need to pull the required columns to put them into SELECT and FROM statements in parenthesis and join them together on the basis of data key relations. In this case, they are best way to provide on the fly views like structures.

Conclusion

Derived tables are the best option to avoid to create views which are not going to be used again and again. They provide a simple way to improve the performance of the SQL Query and SQL Server will take care of the Derived Tables because they stored in the memory and refreshed by the itself.