19. Using Aliases with AS

Core Idea: The AS keyword assigns temporary names to query output columns or source tables, improving readability and disambiguating identical column names across joins.

Column Aliases

            SELECT ProductName AS ItemName, 
                   UnitPrice * Quantity AS LineTotal,
                   UnitPrice * 0.18 AS TaxAmount
            FROM OrderItems;
        

Table Aliases in Joins

            SELECT p.ProductName, c.CategoryName, p.UnitPrice
            FROM Products AS p
            INNER JOIN Categories AS c ON p.CategoryID = c.CategoryID;
        

Aliases with Spaces & Reserved Words

Wrap alias names in square brackets [...] or double quotes if they contain spaces or match SQL keywords:

            SELECT FirstName + ' ' + LastName AS [Full Name],
                   HireDate AS [Start Date]
            FROM Employees;
        

Advanced Deep Dive ADVANCED

Mandatory for Expressions: Computed columns, aggregate outputs, or subquery expressions require aliases if they will be accessed via derived tables or views.

Logical Processing Order: Column aliases defined in SELECT cannot be referenced in the WHERE or HAVING clause because WHERE is evaluated before SELECT. However, aliases can be used in ORDER BY.

Performance Impact: Aliasing is purely a syntactic construct evaluated at compile time; it incurs zero runtime performance overhead.

Self-Joins: Table aliases are strictly required when performing self-joins to reference the same table on both sides of a relationship.