What's the purpose of SQL keyword “AS”?

Question

You can set table aliases in SQL typing the identifier right after the table name.

SELECT * FROM table t1;

You can even use the keyword

Answer 1

Everyone who answered before me is correct. You use it kind of as an alias shortcut name for a table when you have long queries or queries that have joins. Here's a couple examples.

Example 1

SELECT P.ProductName,
       P.ProductGroup,
       P.ProductRetailPrice
FROM   Products AS P

Example 2

SELECT P.ProductName,
       P.ProductRetailPrice,
       O.Quantity
FROM   Products AS P
LEFT OUTER JOIN Orders AS O ON O.ProductID = P.ProductID
WHERE  O.OrderID = 123456

Example 3 It's a good practice to use the AS keyword, and very recommended, but it is possible to perform the same query without one (and I do often).

SELECT P.ProductName,
       P.ProductRetailPrice,
       O.Quantity
FROM   Products P
LEFT OUTER JOIN Orders O ON O.ProductID = P.ProductID
WHERE  O.OrderID = 123456

As you can tell, I left out the AS keyword in the last example. And it can be used as an alias.

Example 4

SELECT P.ProductName AS "Product",
       P.ProductRetailPrice AS "Retail Price",
       O.Quantity AS "Quantity Ordered"
FROM   Products P
LEFT OUTER JOIN Orders O ON O.ProductID = P.ProductID
WHERE  O.OrderID = 123456

Output of Example 4

Product             Retail Price     Quantity Ordered
Blue Raspberry Gum  $10 pk/$50 Case  2 Cases
Twizzler            $5 pk/$25 Case   10 Cases