Level Extreme platform
Subscription
Corporate profile
Products & Services
Support
Legal
Français
SQL with two JOINs
Message
From
04/02/2023 11:38:29
Lutz Scheffler
Lutz Scheffler Software Ingenieurbüro
Dresden, Germany
 
General information
Forum:
Visual FoxPro
Category:
Databases,Tables, Views, Indexing and SQL syntax
Miscellaneous
Thread ID:
01686104
Message ID:
01686114
Views:
26
>>>>>Hi,
>>>>>
>>>>>This is a very newbie question, I know.
>>>>>
>>>>>I am trying to understand the SQL Select that Tamar Granor kindly helped me with. It has two JOIN clauses. And THEN, after the second JOIN, there are two ON clauses. I didn't know you can do it. I thought that ON should follow the JOIN immediately.
>>>>>And looks like the first ON belong to the top (first) JOIN and the second ON belongs to the bottom (second) JOIN.
>>>>>
>>>>>My question is, how does SQL Server (or VFP for that matter) knows which ON apply to which JOIN?
>>>>>
>>>>>TIA
>>>>
>>>>For all I know, there is the following syntax:
>>>>
>>>>Syntax one: The ON follows the JOIN, from top to bottom
>>>>
>>>>SELECT * ;
>>>>FROM Cur1 ;                     && - 
>>>> INNER JOIN Cur2 ;              && - First Join	&& -
>>>>  ON Cur1.Field=Cur2.Field ;    && +            && |
>>>> ;                                              && | Second Join (this is: to the result of the first)
>>>> INNER JOIN Cur3 ;                              && -        && -
>>>>  ON Cur2.Field=Cur3.Field                      && +        && | ....
>>>>
>>>>
>>>>Syntax two: The ONs follows the JOINs, inside to outside
>>>>
>>>>SELECT *;
>>>> FROM Cur1;                                         && -
>>>>  INNER JOIN Cur2;              && -                && -
>>>>  INNER JOIN Cur3;              && - First Join     && |
>>>>  ON Cur2.Field=Cur3.Field;     && +                && | Second Join (this is: to the result of the first)
>>>> ON Cur1.Field=Cur2.Field                           && +
>>>>
>>>>
>>>>You are free to mix the styles. It might create a headache, but is it possible.
>>>>
>>>>Update: tab to space
>>>
>>>Lutz,
>>>
>>>Thank you very much for a clear example explaining my question. The second JOIN syntax was new to me.
>>>But now I see that if there are two JOIN clauses, and the second JOIN syntax is used, the first ON must be applied to the first JOIN.
>>
>>??? The second example:
>>The JOIN between Cur2 and Cur3 (ie the second JOIN) is the first or innermost ON (Cur2.Field=Cur3.Field). It might differ on different engines, but this at least is what VFP is doing. At one moment in time I switched some hundred views from 2 to 1 (while setting up the views to CA) , so there is remembrance of this.
>
>In my case, this particular SQL Select will work only on MS SQL Server. I do not support VFP DB in this particular application.
>And so far, I have been testing the approach suggested by Tamar, it works. Not that I doubt that a young and inexperienced person like Tamar could suggest a wrong approach :)

Nobody is to old to learn new stuff. We are only to old to die young. :)
Words are given to man to enable him to conceal his true feelings.
Charles Maurice de Talleyrand-Périgord

Weeks of programming can save you hours of planning.

Off

There is no place like [::1]
Previous
Next
Reply
Map
View

Click here to load this message in the networking platform