Ian,
Below, Girvin had a warning about joins. Specifically, he was talking
about inner joins.
SQL has different types of joins. Below, I explain them. Note: It has
been some time since I used SQL. I might have errors.
Sorry if you already know this information. But possibly it will be
helpful to other people.
Suppose that we join two tables, called table_a and table_b.
Suppose that we join these two tables on columns column_a and column_b.
Our join statement will looking something like:
SELECT *
FROM table_a INNER JOIN table_b
ON table_a.column_a = table_b.column_b;
Let a be any row in table_a that satisfies the join condition.
Let b be any row in table_b that satisfies the join condition.
Below, the text INNER and OUTER are optional when you type the joins
in SQL.
1. An INNER JOIN gives you rows of the form (a, b).
2a. A LEFT OUTER JOIN gives you rows of the form (a, b) and (a, NULL).
2b. A RIGHT OUTER JOIN gives you rows of the form (a, b) and (NULL, b).
2c. A FULL OUTER JOIN gives you rows of the form (a, b), (a, NULL), and
(NULL, b).
3. A CROSS JOIN is a Cartesian product between the rows of table_a and
the rows of table_b. It gives you rows of the form (aa, bb), where aa is
any row in table_a, and bb is any row in table_b.
Winston
On 01/08/2013 10:55 PM, Winston Chuen-Shih Yang wrote:
Ian,
Below, Girvin had a warning about joins. Specifically, he was talking
about inner joins.
SQL has different types of joins. Below, I explain them. Note: It has
been some time since I used SQL. I might have errors.
Sorry if you already know this information. But possibly it will be
helpful to other people.
Suppose that we join two tables, called table_a and table_b.
Suppose that we join these two tables on columns column_a and
column_b.
Our join statement will looking something like:
SELECT *
FROM table_a INNER JOIN table_b
ON table_a.column_a = table_b.column_b;
Let a be any row in table_a that satisfies the join condition.
Let b be any row in table_b that satisfies the join condition.
Below, the text INNER and OUTER are optional when you type the
joins in SQL.
1. An INNER JOIN gives you rows of the form (a, b).
2a. A LEFT OUTER JOIN gives you rows of the form (a, b) and (a, NULL).
2b. A RIGHT OUTER JOIN gives you rows of the form (a, b) and (NULL, b).
2c. A FULL OUTER JOIN gives you rows of the form (a, b), (a, NULL),
and (NULL, b).
3. A CROSS JOIN is a Cartesian product between the rows of table_a and
the rows of table_b. It gives you rows of the form (aa, bb), where aa
is any row in table_a, and bb is any row in table_b.
Winston
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
Ian Whitfield wrote:
Hi All
Re - My previous post Have been doing some Googling etc and
found the 'Relationships Window' for setting Relationships.
I can get the Window up, select my Tables but it _DOES NOT_ draw any
connecting lines or set any Relationships!!??
Is this another Gotcha of using MySQL and Base together? As it
does _NOT_ seem to work at all!!
I'm using PCLinuxOS 2012, LO Base 3.6.2.2 and MySQL 5.1.55
IanW
Pretoria RSA.
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