Re: [libreoffice-users] Re: Base scenario

Dan Lewis wrote:
>  Comment inline below.
>
> --Dan
>
> On 01/08/2013 07:07 PM, Girvin R. Herr wrote:
> > Ian,
> > Have you actually drawn any relationships?  Base will not do that for 
> > you.  Just adding the tables in the 'Relationships Window' will not 
> > create the relationships automagically.  You must click and hold on 
> > the one table element (remote key) and drag over to the related 
> > table's element (primary key), then release the mouse button.  Base 
> > will then draw a line between the two.  Note, the order of the drag 
> > is important.  It determines the type of join. Joins are confusing to 
> > me too, so I can't help much there.  I had to experiment with the 
> > direction to get it to work right.  I think it was remote key to 
> > primary key, but I am not sure of that any more.
> >
> > Warning!  The way the SQL language is set up, if either of the ends 
> > of a join (relationship) is NULL, then the record will be discarded 
> > and not show up in your result set.  No warnings, no errors.  Data 
> > records will just be missing.  IMHO, this is stupid (my mantra is: 
> > "thou shall not lose data"), but that is how the SQL language was set 
> > up.  So, make sure any joined data elements in all of your table 
> > records are not NULL.  Note that NULL is not zero (0) and 
> > vice-versa!  NULL means that there is no data in the record element.  
> > I use a lot of remote keys in my database main tables that point to 
> > primary keys (options) in other tables.  In those other tables, I 
> > have made it a point to make the data elements of the first record to 
> > be "-", which is my equivalent of unknown, just to have something to 
> > select that is not NULL.  You could probably use a blank (" "), but I 
> > prefer seeing the "-" in forms and reports.  Most times in reports, 
> > it is hard to see anyway.  Seeing the "-" tells me the field is not 
> > NULL.
> > Hope this helps.
> > Girvin Herr
>       These statements about joins do not seem to be quite correct. 
> What you are describing is an Inner Join: you will only see the rows 
> of data in which both the foreign (remote) key and the primary key 
> have a value.
>      Suppose we have two tables A and B and that the foreign (remote) 
> key is in table A and the primary key is in table B.
>     Example 1: table A Left Outer Join table B. The output (result 
> set) for this contains all the fields in table A and their values on 
> the left side of the combined table. The right side contains all the 
> fields in Table B. The rows in which the primary key value matches the 
> foreign key value, data from both table appear in the output. However, 
> where there is no primary key value in table B that matches the 
> foreign key value in table A, all the fields from table B for that row 
> will be NULL.
>      Example 2: table A Right Outer Join table B. The output for this 
> contains all the fields in table B and their values on the right side. 
> For each output row in which the foreign key does not have a value 
> that matches any value of the primary key, the fields in the left side 
> of it will be NULL.
>      Example 3: table A Cross Join table B. This is also referred to 
> as a Cartesian Product. In this case, each row of table A is joined to 
> all the rows of table B. This contains all of the possible 
> combinations of combining both tables. Usually, some rows of the 
> output will have the table A fields all showing NULL while others will 
> have the fields of table B showing all NULL.
Dan,
Your and Winston's explanations of joins cleared up a lot of my 
misunderstanding of them.  You can only get so much from books.  The 
fact that my using an Inner Join requires non-null values to get a 
complete result set makes a lot of sense now.  If I now understand 
correctly, I should be using a Left Outer Join.  That way I always get 
the "table A" data in the result set, but "table B" data may be null if 
there is no match to "table A".  That is the response I was looking 
for.  I just tried it and it does give me the same result set 
information (no records missing) as the Inner Join.  However, I have no 
Null values in "table A".  It may take me a little while to test it with 
Nulls in the Left ("table A") side.  I also recant and apologize for my 
"stupid" remark about SQL losing data.  The reasons for the missing data 
are now apparent to me and that remark was premature. 

While I was doing this join editing testing on my sample query, I took 
time to look at the order of joining in the Query Design window and it 
looks like in order to get the main table (with foreign key) as "table 
A", I had to first click on the "sub" table ("table B") element and then 
drag to the "table A" element.  In other words, it needs to be primary 
key -> foreign key.  My original response to Ian was not sure of this 
drag order.  In any case, the Join Properties dialog and prompt show the 
correct order in the "Fields Involved" window, with "table A" in the 
left column and "table B" in the right column.

Thanks for the help.
Girvin Herr
<snip>


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