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Alias in SQL Server

In SQL Server, an alias is a temporary name assigned to a table, column, or expression in a query to make the query more readable or to differentiate between multiple instances of the same object. Aliases are commonly used in SELECT, FROM, and JOIN clauses. Here are examples of using aliases in SQL Server:

1- Table Alias: 


SELECT e.FirstName, e.LastName
FROM Employees AS e
WHERE e.Department = 'Sales';

In this example, "e" is an alias assigned to the "Employees" table. It allows us to refer to the table using the shorter alias "e" instead of the full table name.

2- Column Alias: 


SELECT SUM(Quantity) AS TotalQuantity
FROM Orders;

Here, "TotalQuantity" is an alias assigned to the result of the SUM() function. It provides a meaningful name for the calculated column in the query result.

3- Alias in JOIN: 


SELECT e.FirstName, d.DepartmentName
FROM Employees AS e
JOIN Departments AS d ON e.DepartmentID = d.DepartmentID;

In this example, "e" is an alias for the "Employees" table, and "d" is an alias for the "Departments" table. The aliases are used to specify the join condition between the two tables.

4- Alias in Subquery: 


SELECT e.FirstName, e.LastName
FROM (
    SELECT EmployeeID, FirstName, LastName
    FROM Employees
    WHERE Department = 'Sales'
) AS e;

Here, "e" is an alias for the subquery. It allows us to treat the subquery as a temporary table and refer to it in the main query.

Aliases are often assigned using the "AS" keyword, but it is optional in SQL Server. For example, you can write FROM Employees e instead of FROM Employees AS e for a table alias.

Using aliases in SQL Server can improve the readability and conciseness of your queries, especially when dealing with complex joins, subqueries, or long table/column names. They provide a way to assign temporary names to objects within the query, making it easier to understand and maintain the code.

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