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How to join 3 tables
Message
From
21/10/2003 19:40:53
Hilmar Zonneveld
Independent Consultant
Cochabamba, Bolivia
 
 
To
21/10/2003 19:14:04
General information
Forum:
Visual FoxPro
Category:
Databases,Tables, Views, Indexing and SQL syntax
Miscellaneous
Thread ID:
00840872
Message ID:
00840897
Views:
14
This message has been marked as the solution to the initial question of the thread.
>>To avoid the .NULL. values, use the nvl() function, thusly:
>>
>>
>>select ..., nvl(b.desc, space(30)) as desc, ...
>>
>>
>>(assuming the description field has a width of 30 bytes).
>
>
>nope, that still gives me the names of all vendors, even though they do not belong to that part.
>sele ;
>      a.pn, ;
>      nvl(b.desc, space(35)) as desc, ;
>      nvl(c.name, space(35)) as name ;
>      distinct ;
>   from ;
>      pnvendor a left join inv b on a.pn = b.pn, ;
>      pnvendor a left join vendor c on a.vendorid = c.vendorid ;
>   where ;
>      "1004" $ a.vendorid .and. ;
>   "04AV205" $ a.pn
>
I am not sure why it works at all. I assume you want to join a with b, and a with c. I would rewrite this as follows:
sele ;
      a.pn, ;
      nvl(b.desc, space(35)) as desc, ;
      nvl(c.name, space(35)) as name ;
      distinct ;
   from ;
      pnvendor a;
         left join inv b on a.pn = b.pn, ;
         left join vendor c on a.vendorid = c.vendorid ;
   where ;
      "1004" $ a.vendorid .and. ;
   "04AV205" $ a.pn
This is not only aesthetic: I omitted the second mention of "pnvendor a". I am not sure whether both versions would produce the same results.

Please note that the LEFT clause will include all data on the left side of the relation, even if there is no equivalent on the right side. This would include all vendors, in your case, even if they have no data on the right side.

If you don't want this, omit the LEFT clause (use only JOIN) on one relation, or both.

HTH,

Hilmar.
Difference in opinions hath cost many millions of lives: for instance, whether flesh be bread, or bread be flesh; whether whistling be a vice or a virtue; whether it be better to kiss a post, or throw it into the fire... (from Gulliver's Travels)
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