Re: How to count from a second table in an aggregate query?

On Apr 15, 2009, at 1:15 PM, Steve Midgley wrote:

> Hi,
>
> I'm trying to figure out how to do something which I'd guess is easy  
> for a sql whiz but has me stumped. I would greatly appreciate any  
> help on this - it's a form of SQL query that I've never figured out,  
> but have wanted to use many times over the years..
>
> I want to generate an analysis report that counts the values in two  
> separate tables. I've been able to accomplish what I want with two  
> separate queries that I then merge together in Excel. Essentially  
> what I need is a "horizontal UNION" statement (or something like  
> that).
>
> I've included some DDL and sample SQL queries that explain what I  
> want better than I can in English, but the general idea is:
>
> get a FK id and count of a certain column in one table, based on  
> some criteria
> -> for each FK id, get the count of a different column in a  
> different table
> Display the counts from both queries side-by-side along with the FK  
> id's in a single result set
>
> Thanks for any assistance on this!
>
> Steve
>
> /*SQL STARTS*/
> drop table if exists contact_log;
> drop table if exists contact_property;
> create table contact_log(id serial NOT null, src_contact_id integer,  
> log_type character varying(63), CONSTRAINT contact_log_pkey PRIMARY  
> KEY (id));
> create table contact_property(id serial NOT null, contact_id  
> integer, property_id integer,
> CONSTRAINT contact_property_pkey PRIMARY KEY (id), CONSTRAINT  
> contact_property_cid_pid UNIQUE (contact_id, property_id));
> insert into contact_log (src_contact_id, log_type) values(1, 'web');
> insert into contact_log (src_contact_id, log_type) values(1, 'web');
> insert into contact_log (src_contact_id, log_type) values(1, 'web');
> insert into contact_log (src_contact_id, log_type) values(1,  
> 'detail');
> insert into contact_log (src_contact_id, log_type) values(1,  
> 'detail');
> insert into contact_log (src_contact_id, log_type) values(2,  
> 'detail');
> insert into contact_log (src_contact_id, log_type) values(2,  
> 'detail');
> insert into contact_log (src_contact_id, log_type) values(2, 'web');
> insert into contact_log (src_contact_id, log_type) values(2,  
> 'foobar');
> insert into contact_log (src_contact_id, log_type) values(3,  
> 'foobar');
> insert into contact_log (src_contact_id, log_type) values(4, 'web');
> insert into contact_property (contact_id, property_id) values(1, 20);
> insert into contact_property (contact_id, property_id) values(1, 21);
> insert into contact_property (contact_id, property_id) values(1, 22);
> insert into contact_property (contact_id, property_id) values(2, 23);
> insert into contact_property (contact_id, property_id) values(2, 24);
> insert into contact_property (contact_id, property_id) values(1, 50);
> insert into contact_property (contact_id, property_id) values(3, 51);
> insert into contact_property (contact_id, property_id) values(5, 52);
>
>
> -- This gets what I want from contact_log
> select src_contact_id, count(log_type)
> from contact_log
> where
> contact_log.src_contact_id in (select contact_id from  
> contact_property)
> and log_type in ('web', 'detail')
> and src_contact_id in (select contact_id from contact_property)
> group by src_contact_id
> order by src_contact_id;
> -- correct output is : 1|5, 2|3
>
> -- This gets what I want from contact_property
> select contact_id, count(property_id)
> from contact_property
> where
> contact_id in (select src_contact_id from contact_log where log_type  
> in ('web', 'detail'))
> group by contact_id
> order by contact_id;
> -- correct output is: 1|4, 2|2
>
> -- THIS DOESN'T WORK (of course - but what would?)
> select src_contact_id, count(log_type), count(property_id)
> from contact_log
> join contact_property cp on cp.contact_id = contact_log.src_contact_id
> where
> contact_log.src_contact_id in (select contact_id from  
> contact_property)
> and log_type in ('web', 'detail')
> group by src_contact_id
> order by src_contact_id
> -- correct output *should be* : 1|5|4, 2|3|2
> /*SQL ENDS*/

First, in that last query, working or not, you don't need the  
"contact_log.src_contact_id in (select contact_id from  
contact_property)" clause as  you've already covered that with the  
join condtion "cp.contact_id = contact_log.src_contact_id".

Anyways, on to your actual question, you can't do that in one level  
from what I can see as the query first does the join and the executes  
the aggregates on the results of the join.  Let's check out the  
results of that join without the aggregates (I'm ignoring the id  
values here since they don't come into play and it will help  
demonstrate what's happening later):

select cl.src_contact_id, cl.log_type, cp.contact_id, cp.property_id
from contact_log cl, contact_property cp
where cl.src_contact_id = cp.contact_id    and cl.log_type in ('web', 'detail');

src_contact_id | log_type | contact_id | property_id
----------------+----------+------------+-------------              1 | detail   |          1 |          20
1 | detail   |          1 |          20              1 | web      |          1 |          20              1 | web
|         1 |          20              1 | web      |          1 |          20              1 | detail   |          1 |
        21              1 | detail   |          1 |          21              1 | web      |          1 |          21
         1 | web      |          1 |          21              1 | web      |          1 |          21              1 |
detail  |          1 |          22              1 | detail   |          1 |          22              1 | web      |
    1 |          22              1 | web      |          1 |          22              1 | web      |          1 |
  22              2 | web      |          2 |          23              2 | detail   |          2 |          23
   2 | detail   |          2 |          23              2 | web      |          2 |          24              2 | detail
 |          2 |          24              2 | detail   |          2 |          24              1 | detail   |          1
|         50              1 | detail   |          1 |          50              1 | web      |          1 |          50
           1 | web      |          1 |          50              1 | web      |          1 |          50
 

That is exactly what gets processed by the aggregates.  Aggregates  
process *a* relation, here the relation produced by the join.  Now,  
that query with the counts makes more sense:

select cl.src_contact_id, count(log_type), count(property_id)
from contact_log cl, contact_property cp
where cl.src_contact_id = cp.contact_id    and cl.log_type in ('web', 'detail')
group by cl.src_contact_id
 src_contact_id | count | count
----------------+-------+-------              1 |    20 |    20              2 |     6 |     6

Joining against a subquery for the second count does the trick:

select src_contact_id, count(log_type), cp.count
from contact_log ,    (select contact_id, count(property_id)      from contact_property      group by contact_id) as
cp
where src_contact_id = cp.contact_id    and log_type in ('web', 'detail')
group by src_contact_id, cp.count
order by src_contact_id
 src_contact_id | count | count
----------------+-------+-------              1 |     5 |     4              2 |     3 |     2

Note that you have to add the count pulled from subquery to the group  
by since at that point it's a constant and not an aggregate function  
anymore, else you get an SQL error in the outer group by clause.

Here's the query without the aggregate in the outer query to help make  
that clear:

select src_contact_id, log_type, cp.count
from contact_log ,    (select contact_id, count(property_id)     from contact_property     group by contact_id) as cp
where src_contact_id = cp.contact_id    and log_type in ('web', 'detail')
order by src_contact_id;

src_contact_id | log_type | count
----------------+----------+-------              1 | web      |     4              1 | detail   |     4              1
|web      |     4              1 | web      |     4              1 | detail   |     4              2 | detail   |     2
            2 | detail   |     2              2 | web      |     2
 

That says, "Give me each src_contact_id and log_type pair from  
contact_log along with the count from contact_property where  
contact_id = src_contact_id".

Erik Jones, Database Administrator
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