On Mon, Jul 28, 2014 at 3:10 PM, Drago, William @ MWG - NARDAEAST <william.dr...@l-3com.com> wrote: > All, > > Can someone tell me what the purpose of line 2 is in the following example? > It seems redundant to me since what is wanted from the Customers table is > specified on line 1. > > > 1: SELECT Customers.CustomerName, Orders.OrderID > 2: FROM Customers > 3: INNER JOIN Orders > 4: ON Customers.CustomerID=Orders.CustomerID > 5: ORDER BY Customers.CustomerName; > > The above example was found here: > > http://www.w3schools.com/sql/sql_join_inner.asp > > > Thanks, > -- > Bill Drago > Senior Engineer
Having read Igor response, I realize that I totally missed the intent of your question. In line one, the "Customers." does _NOT_ necessarily name a table! It could be a table name. Or it could be an alias name. An alias name version of the above might be. SELECT c.CustomerName, o.OrderID FROM Customers AS c INNER JOIN Orders as o ON c.CustomerID=o.CustomerID ORDER BY c.CustomerName; One reason to do this is simply to shorten what needs to be typed in. But another reason is when you refer to a single table (or view) multiple times in a single SELECT. This is often called a "self join". A bit on an explanation on this is here: http://www.tutorialspoint.com/sql/sql-self-joins.htm But basically an alias is needed in this case to distinguish which instance of the table is being used for what. Example: You have a table with names and ages. You want an age report. Who is older than whom? SELECT a.person as olderPerson, b.person as youngerPerson, a.age-b.age as ageDifference FROM people AS a INNER JOIN people as b where a.person=b.person and a.age > b.age; You need the aliases "a" and "b" to compare the table with itself properly. -- There is nothing more pleasant than traveling and meeting new people! Genghis Khan Maranatha! <>< John McKown _______________________________________________ sqlite-users mailing list sqlite-users@sqlite.org http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users
