What is SQL function and User Defined Table?

SQL functions and user-defined tables are two important concepts in SQL that help in organizing and manipulating data effectively.

SQL Functions

SQL functions are predefined operations that can be used to perform calculations, manipulate data, and return results. These functions can be used in SQL queries to perform various tasks such as mathematical calculations, string manipulations, date and time operations, and more. SQL functions can be categorized into different types, including aggregate functions, scalar functions, and window functions.

For example, the SUM() function is an aggregate function that calculates the sum of a set of values in a column. The UPPER() function is a scalar function that converts a string to uppercase. The ROW_NUMBER() function is a window function that assigns a unique number to each row in a result set.

User-Defined Tables

User-defined tables, also known as temporary tables or derived tables, are tables that are created and used within a specific SQL query. These tables are not stored permanently in the database but are created on-the-fly to perform complex queries or intermediate calculations.

User-defined tables are useful when we need to break down a complex problem into smaller, more manageable parts. They allow us to create temporary structures to store intermediate results and perform further operations on them. User-defined tables can be created using the CREATE TABLE statement and can be used in subsequent SQL statements within the same query.

For example, we can create a user-defined table to store the result of a subquery and then join it with other tables to retrieve the final result.

CREATE TABLE temp_table AS
SELECT column1, column2
FROM table1
WHERE condition;

SELECT *
FROM temp_table
JOIN table2 ON temp_table.column1 = table2.column1;

Thanks SUNIL R S

User-defined functions are nothing but methods (like in c#) that can return a scalar value, or a  table, you can use a function directly in your select query unlike Stored procedure 

CREATE FUNCTION dbo.GetEmployeeDetails (@DepartmentID INT)
RETURNS TABLE
AS
RETURN
(
    SELECT EmployeeID, FirstName, LastName
    FROM Employees
    WHERE DepartmentID = @DepartmentID
);

A User-Defined Table Type is a user-defined data type that represents the structure of a table.

CREATE TYPE dbo.EmployeeTableType AS TABLE
(
    EmployeeID INT,
    FirstName NVARCHAR(50),
    LastName NVARCHAR(50)
);