Yes it is a good idea to store in a table information about which other
tables should be searched.
I don't know how these queries can be made using only SQL. I think that it
could be made in the programming language you use.
For example, first get the list of tables that should be searched (from
that index table), then create that SQL query that uses union using only
those tables.
It shouldn't be too hard to do.
For example, after searching the index table, it could return that you need
to search in the books and CDS tables.
Then you could create that sql query like:
$sql = "";
for("books", "cds") {
$sql .= join " union ", "(select id, title, from $_)";
}
$sql .= " where ... order by ... limit ...";
So the sql query will search only in the needed tables.
Octavian
----- Original Message -----
From: "Ed Lazor" <[EMAIL PROTECTED]>
To: "'Octavian Rasnita'" <[EMAIL PROTECTED]>; <[EMAIL PROTECTED]>
Cc: <mysql@lists.mysql.com>
Sent: Wednesday, June 27, 2007 8:02 PM
Subject: RE: select statement with variable for table_reference?
Hi Octavian,
First, thanks for helping out. I really appreciate it. Thanks to you
also
Randall.
I am not sure I understand what you want.
If you want to search for all cds, and books, and dvds based on a certain
criteria, you can use that method I've told you about.
Randall said it best. I have one table that has information about what
other tables to search in.
One table serves as an index of what's in a user's inventory while the
actual product information resides in other tables.
The user inventory table has fields for user_id, database_id, table_id,
and
record_id.
There are also two other "helper" tables. One table (inventory_databases)
contains a list of databases with their id and name. Another table
(inventory_tables) contains a list of tables with their id and name. Both
of these tables help map from the user's inventory to where product
information resides.
An example record from the inventory table would have data like this:
user_id 33
database_id 1
table_id 1
record_id 234234
I can look up the name of the database using database_id in the
inventory_databases table. I can look up the name of the table using
table_id in the inventory_tables table.
All of the other product tables have an id field that corresponds to the
record_id.
Back to the example above, database_id 1 is the products1 database and
table_id 1 is the books table. That means user id 33 has the book id
234234
in products1.books.
Ok, that describes what I'm working with. As for what I'm trying to
accomplish, I'm trying to reduce the number of queries required for
pulling
together basic information about the user's inventory. Why is that?
Well...
Right now I run one query the inventory table for all information specific
to the user. Next, I use this information and run additional queries to
get
the actual product information. If the user has 1000 items in their
inventory, I end up having to run 1 + 1000 queries. This can't be helped
in
situations where I refer to fields that are unique to each type of item
(aka
table). It seems like I should be able to avoid this though when dealing
with a common field like title. I'm just not sure how to go about it.
Using UNIONS is the only single query to work so far. Like I mentioned
though, this requires a UNION for every table that product information is
being stored in. If the first table can tell us where the data resides,
it
seems like we can use it to reduce the number of UNIONS required.
Randall, your use of prepared statements and stored procedures seems like
a
good approach. It might provide a way to dynamically generate the product
query. When I look at this:
SET @strOtherTable = (SELECT other_table_name FROM first_table WHERE
...);
SET @strSQL = CONCAT("SELECT ... FROM ", @strOtherTable, " WHERE...");
The first thing I automatically think of is that the first select will
very
likely have more than one record in the result set. Is there a way to
loop
through the results to concatenate everything into the set of UNIONS?
Ed
May you want sometimes to search only in fewer tables that you know
before
making the query? If yes, then you can create more separate queries that
search only in those tables.
If you want to search only in the tables that have data about you want to
search, you can't do it, because you don't know if those tables contain
what
you want before searching in them.
But if those tables have well defined indexes on the keys you are
searching
for, that search will be very fast, especially if some of the tables
don't
contain records that match your criteria.
I hope I understood correctly what you want.
If you just want to specify a search criteria for each table separately,
you
can do it for each table in the (select ... where ...) and if you want to
specify a search criteria for all the records of those unions, you can do
it
in a final where ... that's outside of those params.
Octavian
----- Original Message -----
From: "Ed Lazor" <[EMAIL PROTECTED]>
To: "'Octavian Rasnita'" <[EMAIL PROTECTED]>; <mysql@lists.mysql.com>
Sent: Tuesday, June 26, 2007 11:37 PM
Subject: RE: select statement with variable for table_reference?
> Ok, I used your approach like this:
>
> --------------------------------------------------------------
> select i.scanned_barcode, v.title from inventory as i
> left join version as v on i.record_id = v.id
> where
> i.database_id = '1' AND i.table_id = '1' AND
> i.user_id = '33' and category_id = '766')
>
> UNION
>
> (select i.scanned_barcode, v.title from inventory as i
> left join amg.dvd as v on i.record_id = v.id
> where
> i.database_id = '2' AND i.table_id = '3' AND
> i.user_id = '33' and category_id = '766')
>
>
> order by title DESC
> --------------------------------------------------------------
>
> It "works" like you're suggesting. I have to add a union for every one
of
> the tables data is being stored in. That means I end up selecting
> something
> from every product table, regardless of whether the user actually has
> something in there or not. Improving on this idea would be finding a
way
> to
> just query the relevant tables... some sort of conditional union. Any
> ideas?
>
> -Ed
>
>
>
>> -----Original Message-----
>> From: Octavian Rasnita [mailto:[EMAIL PROTECTED]
>> Sent: Tuesday, June 26, 2007 1:02 PM
>> To: Ed Lazor; mysql@lists.mysql.com
>> Subject: Re: select statement with variable for table_reference?
>>
>> I am using the following method for doing this, but I am sure it is
>> not
>> the
>> best one:
>>
>> (select id, title, author, 'book' as type from books)
>> union
>> (select id, title, author, 'cd' as type from cds)
>> union
>> (select id, title, author, 'dvd' as type from dvds)
>> where ...
>> order by ...
>> limit ...;
>>
>> Octavian
>
>
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