The LA Fox Developer Newsletter

April 1998

Outer Joins (Con’t from page 4)

Join - that is, you will get all the records from the second (right) table and only matching record from the first (left) table. In our example the end result will be

A Arnon Gal-Oz

BTom O’Hare

C Michel Fournier

D Scot Becker Elliot

FULL OUTER JOIN - as its name implies this will have the non- matched and matched records from both table. In our example the end result will be

AArnon GaI-Oz

B Tom O’Hare

C Michel Fournier

0 Scot Becker

E Paul

Elliot

Note that we did not get a key for “ELLIOT’ in the RIGHT and JOINs this is because out orifinal SELECT statement is SELECT A.Key and A does not have a key for Elliot to solve this in the RIGHT JOIN all we have to do is change the SELECT to B.Key... instead, but this will not work in the case of the FULL join - because then we.will not have a key for “PAUL”, if we do a FULL JOIN we need to use a HF tro the fields in the JOIN condition (if we want them as part of the result) in out example it will look like:

SELECT llF(A. Key#” “,A. Key, B. Key),A. name,B.fname FROM A FULL JOIN B ON A.Key==B.Key

And naturally u can play with this and with FoxPro ability fo sub-queries to get only nomatching record etc. etc.

Polymorphism (Con’t from page 4) polymorphism work any differently than this article is describing? Well, what could happen (and does sometimes in other languages) is that without polymorphism, if you sent a double click message to the child’s form, the base class code would fire (not the child’s), which is not what you want. The fact that polymorphic behavior is automatic(that is, you don’t have to create any special functions for polymorphism to work correctly as you do in other languages like C++), is a nice benefit of using VFP.

Okay, back to the code above. If you change the CREATEOBJECT to:

oExample=CREATEOBJEcT(”PARENT’)

oExample.Show()

READ EVENTS

and then double click the form, you’ll see the WAIT WINDOW for the parent. Sounds good, right? Pretty easy, right? That’s all there is to it!!!

Omitting code in a class

What would happen if we used:

oExampte=cREATEOBJEcT(”cHILD”)

/oExample.Show()

READ EVENTS

and we left out the procedure for the DBLCLICKO procedure? Simple. The code for the next member up the class hierarchy is called. Thus, you’d see the WAIT WINDOW for parent again. If there is no code in the method code for the object you’re double clicking VFP looks up the class hierarchy for method code it can execute. When VFP finds method code for double click up the hierarchy it executes it!

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