Обсуждение: counting bools in a complex query

Hi,

I think I've created a monster...

Working on an email system I have the following:
Table    = usermail
+----------------------------------+--------------------------+-------+
|              Field               |              Type        | Length|
+----------------------------------+--------------------------+-------+
| contentlength                    | int4                     |     4 |
| folder                           | int4                     |     4 |
| flagnew                          | bool                     |     1 |
etc...

And:
Table    = folders
+----------------------------------+--------------------------+-------+
|              Field               |              Type        | Length|
+----------------------------------+--------------------------+-------+
| loginid                          | varchar() not null       |    16 |
| folderid                         | int4 not null default (  |     4 |
| foldername                       | varchar()                |    25 |
etc...

So each email message has an entry in usermail, and each mail folder has
an entry in folders. I need to extract the following info:
foldername, number of messages in that folder, number of messages in that
folder with flagread set, total size of all the messages in each folder

Since postgres does not appear to support outer joins, I've come up with a
really icky query that almost does what I want:

SELECT folderid,foldername,count(*),sum(contentlength)  FROM usermail,folders  WHERE usermail.loginid='michael' AND
 folders.loginid=usermail.loginid AND        usermail.folder=folders.folderid GROUP BY folderid,foldername 
 
UNION SELECT folderid,foldername,null,null FROM folders  WHERE loginid='michael' AND        folderid NOT IN
(SELECTfolder FROM usermail WHERE loginid='michael');
 

WHEW!

folderid|foldername      |count|    sum
--------+----------------+-----+-------     -4|Deleted Messages|  110| 245627     -3|Saved Drafts    |     |
-2|SentMail       |    7|  10878     -1|New Mail Folder |   73|8831226      1|OOL             |    7|   8470
 
etc...

My final problem is to count all the messages with flagnew set to true.
The only way I can think to do this is to convert the bool value to a 1 or
0 (which I think should be a standard conversion anyway) and run a sum()
on them.

Unless anyone can come up with a better way to do this, What is the best
way to implement a conversion from bool to int?

-Michael

At 02:31 +0300 on 14/07/1999, Michael Richards wrote:


> My final problem is to count all the messages with flagnew set to true.
> The only way I can think to do this is to convert the bool value to a 1 or
> 0 (which I think should be a standard conversion anyway) and run a sum()
> on them.
>
> Unless anyone can come up with a better way to do this, What is the best
> way to implement a conversion from bool to int?

Depends on your version. If you have 6.5, you can probably use COALESCE or
CASE. In previous versions, I would define an SQL function. The first one
that comes to mind would be:

CREATE FUNCTION to_int( bool ) RETURNS int4 AS
'SELECT 1 WHERE $1 UNION SELECT 0 WHERE NOT $1';

Another approach would be the following:

CREATE FUNCTION only_true( bool ) RETURNS bool AS
'SELECT $1 WHERE $1 IS NOT NULL AND $1';

This would return true when the argument is true, null in all other cases.
You can use this with count rather than sum.

Of course, you could decide that you would fill the flagnew with either
true or null, instead of true and false. If you do, you could do a count
without the above function. You could get the true/false by asking whethe
flagnew IS NOT NULL.

Finally, the UNION is the way to go when you want to implement an outer
join, but using NOT IN is not recommended, because it is inefficient. A
better subquery would be:

NOT EXIST (  SELECT  * FROM usermail  WHERE loginid='michael'    AND folder = folderid
)

Herouth

--
Herouth Maoz, Internet developer.
Open University of Israel - Telem project
http://telem.openu.ac.il/~herutma