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Old 05-15-2009, 03:33 AM   #1
Thor_2.0
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Wink DB question...


Hi,

Good database design suggests that data should NOT be repeated across a DB system, so, the name of a product should only appear in ONE table only and a join has to link the tables in a query.

Assuming the following

Quote:
CREATE TABLE tblTags
(
ID INT( 5 ) NOT NULL ,
tagtext VARCHAR( 15 ) NOT NULL ,
PRIMARY KEY ( `ID` )
)

CREATE TABLE tblTagPos
(
ID INT( 5 ) NOT NULL ,
tagpos VARCHAR( 15 ) NOT NULL ,
PRIMARY KEY ( `ID` )
)
a select to find out where what tag is (a label on a warehousing shelve) one would use this:

Quote:
select tblTags.ID, tblTagPos.ID, tblTags.tagtext, tblTagPos.tagpos where tblTags.ID = tblTagPos.ID order by tblTag.ID
That works in a DB where the two sit in the (physically) same database, how would I do that in Java, knowing that a Connection A was set to a common database and Connection B "looks" to the actual "data" database, the tags never change, so I'd not really HAVE to copy this table to the data but leave it in the core...

Preceding the table names with the database name is a known trick to me, but that may fail, as tblTags would be in "core" and accessible via Connection A, and tblTagPos is in the "data" database and is subject to change (the positions on the shelves can change)...

It CAN be done, but (as far as I can see) it would involve two fetch runs, one to get the positions, and one to fetch the corresponding tag texts, about as efficient as a belgian politician

Hints? Tips anyone?

Tnx

Thor
 
Old 05-15-2009, 03:40 AM   #2
Thor_2.0
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Addendum to my first post here:

I know that it is possible to link tables across databases (some "others" do can do that) - that meant creating a "symlink" from database A to database B, can mySQL do that?

That...would be T-H-E solution!!

Security permitting, of course.

Thor
 
Old 05-18-2009, 06:03 AM   #3
Guttorm
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If you are using MySQL, look at the federated engine, which is available in the newest versions.

http://dev.mysql.com/doc/refman/5.1/...ge-engine.html

The symlink approach could work, but locking would be an issue. You would have to run both servers with "--external-locking" but then both would be slowed down a lot.

Last edited by Guttorm; 05-18-2009 at 06:06 AM.
 
Old 05-19-2009, 01:00 PM   #4
Thor_2.0
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Big tnx!!

The "shared" database would be for read-out only...so locking is not that much of an issue, I guess, it just seems a bit silly to fill up several databases with identical data that only needs to take space once, and can therefore be administred centrally...

Let me have a look at the link you gave!

Grtz!

Thor
 
  


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